Thanks for the feedback !
timmaah, your reply looks like it may nail the solution I'm after. I hadn't spotted that, thanks for pointing it out.
ckiick, thanks for the info on how the millis() function works. The name seems misleading to me. timmaahs' solution should deal with the overflow issue, all I have to work out is some algorithm to give me a millisecond conversion (or as near accuracy as I can hope for).
I'm new here but this already strikes me as a very friendly and helpful place.
Many thanks ..... Matt