Thanks for the link, Adrian.If they are BSAC instructors, they can only instruct in a BSAC branch or school.
With regards to welfare issues (CRB checks are NOT necessarily compulsory) have a look at the BSAC Policy WELFARE OF VULNERABLE PEOPLE in Snorkelling and Scuba Diving - BSAC
This isn't about training at all, as training is not done in any way in the club. It is simply aobut how we manage club diving, whether it is shore based, or on boats, and minors. We run dives which are aimed at less experienced divers, so more experienced divers are encouraged to go along and assist, but that is more about support. and making sure that both the young people and the divers are protected.