Thank you for the reply Mike
I used plenty of isopropyl to clean it, and tested all the tracks and joints with surface corrosion, they're all fine. I've replaced the M54513 to no avail, I'll do the BA618 as soon as I can get a socket for it, but I've got a sinking feeling that it might be the 81c55 that's the root of the problem. Fingers crossed that it's not, but i may well be looking for advice on that soon.
Thanks again
Andrew