Magistrates courts UK, duty solicitors and legal aid...please help!?

You have no standing in this case, he needs to inform them if he is going to change solicitor, he's very unlikely to get a barrister at mags court any local solicitor will represent him. He might be better sticking with whoever was duty, as they tend to be the ones that regularly appear at mags (when not acting as duty solicitors) they know the mags, the prosecutors all the other defence solicitors and all the court staff. Nobody is going to accept instructions from you, this is his case, he needs to get off his backside and do this himself, it's not hard. Address it to the, Clerk to the Justices , at whatever address court is (that is where the office will be)and start Dear Sir.