Well I might suggest people look
here for suggestions for accountants, or at least the criteria to apply if you want to find your own, but some would no doubt accuse me of promoting stuff...
To be fair, I know a few High St accountants that do speak IR35, S660 and all the other things quite well. The trouble is differentiating them from the ones that only understand "real" small businesses; the real trouble is you have to use them for a while to find out how good they really are, by which time it may be too late!
Only real way to be safe is to use reccommendations from someone in the same line of business as yourself. And even then, be very diligent; if they get it wrong, it's
your problem not theirs.