Just to answer my own question I found this good little shop for the above today and availed myself of the parts for a full electrical service (plugs/codesor/distributer cap/ht leads and a new plug spanner) all for just 800Baht. The guy knew the engine from the make of car (and my wreck is 13 years old and not terribly common here) - I was impressed. They do not speak any English, so I wrote down what I wanted i Thai script before I went to make it easier all round.

Anyway, as to where this goldmine is..CM Train Station, with your left shoulder pointing at Thomas the Tank Engline, walk on down the road away from town (I guess its North-ish)...go past the Post Office and the petrol station. When you come to the 7-11 on the right, its directly opposite on the left (sun bleached red boarding over the top) - great place, knowledgable, didn't rip me off and service with a smile (he was surpised I was going to service the car myself, he said most Thais can't do it, but Farang can...and then laughed).

Oh and by the way, there is a little shop before the post office where they cut keys while you wait - very cheap, again doesn't rip farangs off, and asked me to come back if I had trouble with the keys - one was a security key that's very hard to copy, he did it (mostly by hand!) and it worked fine.