Thank you all for your excellent tips on solving my problem. I've attached the spreadsheet I am working on but, despite applying the relevant time formatting to the cells, the total refuses to sum up - typing them in manually from scratch works though (the data was exported from an accounts package so perhaps there's a clue there somewhere..).
Derk, your answer looks great but not too sure how to incorporate both formulas tidily.
Once again, thanks to all you guys for your help, it is much appreciated.
Johnners