Hi Joe
This is surprisingly poorly documented. Manual for Streets and DMRB duck to issue, referring only to bus lay-bys.

However, both TfL and what is now the Northern Ireland Department for Infrastructure have on-line design guides that largely agree.

Based upon a 12 m long bus, they say the normal length is 23 m, the absolute minimum 19 m (if there is no adjoining parking), but that 37 m should be allowed for manoeuvring in and out if there is adjoining parking.