Thanks guys for taking the time to reply. I did try Excel but whatever I did ended up a bit clumsy. I knew the arithmatic but didn't have the knowledge of Excel to make it work well.
In the meantime, I have found a nice piece of shareware (although a tad expensive I think) which does exactly what i want at
http://home.nycap.rr.com/aotw/timecalc.htm
If I can round to it, I might just get my logbook added up before the evaluation period is over
Thanks again folks