As I understood it the tester has to be convicted of an offence? If it's something like theft or assult then vosa consider removing them from the scheme. But as far as drugs go, I think it's treated like alcohol? if you get banned from driving for a Drink related offence then you don't automatically get removed from testing ( i know a guy that's been done for DD more than once and is still a tester) so i would guess that unless the powers that be consider that they're a security risk (dodgy passes in return for supply of illegal substances) they'll not get any action against them. That said the firm they work for have a duty of care to thier customers and if they consider them a risk to safety them they should take action.