Blimey! That is very complicated, got lost in there quite quickly...
Thank you Soft sign, that is exactly the sort of information I was hoping to find. I didnt expect it to arrive so promptly! You seem to understand the problem very accurately; are you a software enginmeer or programmer of some sort? I'm not, I'm just following an idea.
Anyway, thanks for the replies, I can get on with designing now...