Even though the “theme” is pretty obvious, on immediate inspection of the clues, the category itself (boys’ names) is pretty expansive. So it still took me a few minutes to figure out the last couple of four-letter ones.
Across
1 ROB(ER)T
4 BRAD,SHAW – the eponymous post-war “Bible” of railway schedules. I think its disappearance more or less coincided with the rise to notoriety of the “British Rail sandwich”, sometime in the 60s/70s.
9 G(ERA)LD – the “Good Lord” container had me initially putting in Geramy. But it was tentative and soon corrected, fortunately.
12 NU,MIN(O)US – “new” + love in minus
15 LIAM (mail) rev
20 TI,MOTHY – I couldn’t help but enjoy the neologism mothy for “insect-like”.
21 JOHN – ref J. Bull
26 (-p)ENTANGLE
30 S,TRIDENT – I still can’t make up my mind whether or not I approve of these stuttering clues (“s-submarine weapon”). Everything in moderation I suppose….
31 AL(B)ERT
Down
1 RE(GIN)AL,D(-ig)
2 BE,RIMBAU(-d) – this “Brazilian instrument” was unknown to me but, having spotted Rimbaud early doors, it was a pretty confident guess.
3 ROLAND – ref the old knock-knock joke…Who’s there?….Roland Butter.
5 (-a)RYAN – the last one to go in for me.
7 HU(BER)T – The most obscure of our three Berts today. “Last month’s end” = (Novem)BER. In his youth, Smiffy lived in the only parish in the UK named after St Hubert.
17 BILL,FOLD – a tricksy one, as (uniquely for this puzzle) the “boy” here is only part of the wordplay, rather than the entire answer.
18 LON,GACRE – (grace)*. An &-lit, supposedly. Although I don’t recall it being synonymous with “grace”.
24 SA(MU)EL – Mu in leas(rev)
27 SEAN – hidden. But took me an inordinate amount of time to spot, because I became mildly obsessed with the (superfluous) word “discipline” in the clue.