Ifor, known for tight but always very fair clueing, and often towards the challenging end of the spectrum.
Preamble: Ifor’s two collaborators X and Y have contributed all across clues bar those for the top and bottom rows. X’s ten (each in a different row) comprise definitions of three related words in arbitrary order, the most suitable one being the entry. Y’s ten are normal, but their answers must be changed to new words in one of two ways to fit their slots. There are five of each type, alternating in clue order. In all cases lengths specify the space available. Each collaborator must be named under the grid; the single letters derived from their respective entry methods will assist.
An asymmetric grid, and mysterious goings-on with the across clues, so I made a start on the downs. Solved about half a dozen here and there, so time to tackle the acrosses. With 1d ASCORBIC & the normal 1a APAY falling easily, my attention was naturally drawn to the top left quadrant. ORA|ORAL|LORAL for 15a and SEAR|SEGAR|SEGGAR for 9a helped with a couple more down answers, and then I managed to make sense of 20a BALTI(C): “confused” was the anagram indicator (not “at sea”, with “at” being part of the wordplay and “sea” the definition) – and the answer had to lose its last letter before entry. 
With that, plus the rubric (Y’s answers must be changed in one of two ways to fit) together with the title of the puzzle (Bed and Breakfast), I immediately thought of PROCRUSTES. By way of confirmation, the C lopped from BALTI(C) was in about the right place in the across clues to generate the the C of PROCRUSTES.
So I knew how Y’s answers had to be treated but it was a little while before I fully twigged how the answers to X’s triplets were related: one answer was too short by a single letter, another was too long by the same letter, and of course, the remaining answer was ‘just right’. And the concatenation of those letters spelt GOLDILOCKS. But it was only right near the end that I realised that the Procrustean stretching was effected by doubling one of the letters already in the answer, not merely by adding another one somewhere.
I don’t enjoy a puzzle so much when the penny (or ha’penny in this case) has dropped so early on, but heigh-ho. And I kind of ran out of steam when it came to completing the wordplay for 3 down answers in the top right quadrant – thanks go to kenmac for rescuing me there. Elsewhere, I’m still not really sure which of the crustaceans in 32a (crab or shrimp) goes with ZOEA and which with ZOOEA.
Thanks Ifor, highly inventive: with all the constraints you set yourself the end product brings to mind the work of the Oulipo group – though of course, this puzzle is not in French!
| No. | Clue | Answer(s) | X | ±Y | Wordplay |
| Across | |||||
| 1 | Satisfy once assistant enters word of agreement (4) | APAY | PA ((personal) assistant) in AY (yes, word of agreement) | ||
| 4 | Rotten “Hearts of Oak” for the locals put on in clips (8) | DADDOCKS | ADD (put on) in DOCKS (clips) | ||
| 9 | Dry up | smoke | box to bake pots (5) | SEAR|SEGAR|SEGGAR | G | – – – | |
| 10 | Rogue heartlessly paraded, maybe wrapped in sackcloth (7) | DRAP[P]ED | +P | [PAR(a)DED]* | |
| 12 | Heap of waste | noise accompanying rebound | expressing disapproval (5) | BING|BOING|BOOING | O | – – – | |
| 14 | More accomplished liars unmasked (4) | ABLE(R) | –R | (f)ABLER(s) (liars) | |
| 15 | Mouths | verbal test | of side of head (4) | ORA|ORAL|LORAL | L | – – – | |
| 17 | Excavate hollow space, keeping surface (6) | STO[O]PE | +O | S(pac)E around TOP (surface) | |
| 18 | Reserve | Aussie reject | took great risks (4) | ICE|DICE|DICED | D | – – – | |
| 20 | Confused public not turning up and at sea (5) | BALTI(C) | –C | [PUBLIC ¬ UP< AT]* | |
| 22 | Polish sweet | pie from Moscow, | provisions for journey (5) | PIROG|PIROGI|PROG | I | – – – | |
| 24 | Some in quarantine concerning islands (5) | AR[R]AN | +R | (qu)ARAN(tine) | |
| 26 | Markets | commercial spread | of flowers (5) | FORA|FLORA|FLORAL | L | – – – | |
| 27 | Island vintage characterised by taking turns (4) | CORF(U) | –U | CRU (vintage) with OF (characterised by) alternating | |
| 28 | Fools repeatedly discovered blank tax returns (6) | ASSES[S] | +S | ((b)l)A(n(k)) SESS< (tax) | |
| 33 | Immature hermit | growing shrimp | plants to make meal (4) | ZOOEA|ZOEA|ZEA | O | – – – {not sure which crustacean defines which answer} |
|
| 34 | Early Scottish Widows’ pension plan naturally closes (4) | ANNA(T) | –T | (pl)AN NAT(urally) | |
| 35 | Indian women’s body covering | Hindu festival | of bile (5) | CHOLI|HOLI|CHOLIC | C | – – – | |
| 37 | Children among the worst parts in abnormal cravings (7) | LE[E]CHES | +E | CH(ildren) in LEES (worst parts) | |
| 38 | Hacker, | perhaps one dispatching | ear to Robert Knox (5) | KICKER|ICER|ICKER | K | – – – | |
| 39 | Company | scratches | debt incurred (5) | CORE|SCORES|SCORE | S | – – – | |
| 40 | Gives up codes, ignoring old cipher as soon as put in place (7) | CONCEDE(S) | –S | CODES ¬ O (0, old cipher) around ONCE (as soon as) | |
| 41 | Accept fresh tenancy to rent (6) | ATTORN | AT (to) TORN (rent) | ||
| 42 | Native doctor managing to lose because of wound (6) | MGANGA | [MANAGING ¬ IN (because of)]* | ||
| Down | |||||
| 1 | Acidic or basic fluid containing carbon (8) | ASCORBIC | [OR BASIC]* around C(arbon) | ||
| 2 | Pinafore without national following playing in your average US city (6) | PEORIA | [PINAFORE ¬ N(ational) ¬ F(ollowing)]* | ||
| 3 | Chief officers not quite rendered stupid by horror (5) | AGHAS | AGHAS(t) (rendered stupid by horror) | ||
| 4 | Old sinks in room welcomed by gods in tomb entrances (6) | DROMOI | ROOM with O(ld) later in DI (gods) | ||
| 5 | No more casually missing hospital opening (4) | ADIT | HAD IT (no more, inf.) ¬ H(ospital) | ||
| 6 | Crease torn end for end (6) | DAGGER | RAGGED (torn) with ends swapped | ||
| 7 | Gem reflected polish with stone for some second to none (4) | OPAL | LAP (polish with stone, rare) (n)O(ne) all< | ||
| 8 | Tangle line in chance to catch on Tweed (4) | KELP | L(ine) in KEP (chance to catch, Scot & N Eng) | ||
| 11 | Rum interbreeding population repeatedly contracted medical condition (8) | DEMERARA | DEME (interbreeding population) 2×RA (Rheumatoid Arthritis, medical condition) | ||
| 13 | Squeeze between vehicles travelling in either direction (5, 2 words) | NIP IN | palindrome | ||
| 16 | Will’s sign of contempt provided over crib’s failure within play (4) | FIGO | IF< (provided) GO (failure within play, cribbage) | ||
| 19 | Boxed in salt before onset of decay (8) | CORNERED | CORN (salt) ERE (before) D(ecay) | ||
| 21 | Seam hardly deviated after missing hem (4) | LARD | [(h)ARDL(y)]* | ||
| 22 | Church must take precedence before the Passover (5) | PASCH | PAS (precedence) before CH(urch) | ||
| 23 | Join pilot and the other people shortly, having caught up (7) | CONNECT | CONN (pilot) ETC (and the other people) with C(aught) earlier | ||
| 25 | Keeper of stock organised food leaving hub (7) | RANCHER | RAN (organised) CH(e)ER (food) | ||
| 26 | Compassionate service for the elderly on line across Glasgow (7) | FEELING | FEE (service, obs) LING (line, Scot) | ||
| 29 | Infrequently drenched section of wood (6) | SOAKEN | S(ection) OAKEN (of wood) | ||
| 30 | Float a plank that’s sawn up (5) | BALSA | A SLAB (plank) all< | ||
| 31 | Short straight line, maybe on a subroutine (5) | MACRO | MACRO(n) (straight line) | ||
| 32 | Confidential seller’s opinion hidden by crowd’s uprising (5) | BOSOM | SO (seller’s opinion) in MOB< (crowd) | ||
| 36 | Region spoken of as warmer and drier (4) | AREA | homophone AIRIER (warmer and drier) | ||
I got onto Ifor’s wavelength this time (more successfully than with French balloonists), making this a proper Goldilocks puzzle. I didn’t fully get the cleverness of the Procrustean stretching, nor that the Goldilocks answer was always the middle one, until being enlightened by the blog; so much unappreciated ingenuity from setters! As HolyGhost implies, the ZO(O)EA answers feel slightly unsatisfactory, because they are the same basic word with the same basic meaning spelled slightly differently, but given the theme’s constraints it’s a minor quibble. Many thanks to Ifor and HG.
Very enjoyable while attempting to work out what games Ifor was playing; once understood, with Goldilocks and Procrustes discovered, I found I still have a large number of unparsed and unsolved clues, and a complete muddle in the row with two ‘y’ clues. I did have a second wind this week, but the top right still never fell. Tough one, thanks to Ifor and HG.
Properly tough, especially as all the Xs were in different rows but the Ys were not! Having checked off ORAL as an X I was 100% convinced the ZOEA answer must be a Y so was stuck on that for some time.
A good challenge and it was interesting to make Procrustes’ acquaintance, though thankfully not in person!
Lots of “phone a friend moments” but a well constructed puzzle
Bet I wasnt the only one looking for bears but the actual answer
was better still and only cruel to words!
Thanks all
A belated word of thanks to HG for the usual detailed and accurate exposition. Private correspondence suggests that you were unusually perspicacious in spotting the theme(s) so early in the solve. Thanks also to those commenting. A few thoughts – I agree that ideally the zoea clue would have resembled the others in using three different definitions, but needs must. And I would have liked to have had exactly two acrosses (one X, one Y) per row, but again the exigencies of grid construction wouldn’t allow it.