Arrrrrrrrrrrrrrrrrrgggggggggggggggggghhhhhhhhhhhhhhh.... well I decided to take the plunge and arse around with the surface mount chips... upon closer inspection it appeared that the MAX232 was getting an intermittant input from the PIC. Both ends of the connection between TX pin of the PIC and T1IN were linked on the reverse layer of the board... and the connection to the via appeared to be duff on the PIC side of the connection... so I've fiilled it with solder and can't get the connection to fail...
Now all I have to do is convince myself that the other things on the board were also a problem and I've not completly wreaked it for no reason.... buuuuuuuuugggggggggggggggggggggggeeeeeeeeerrrrrrrrrrrrr.