If you want decimal hours returned, use:
=(b1-a1-c1)*24

Format as General, or a number.

Regards,
Fred.

"Jon" wrote in message
...
I would like to create the following formulae:

A1: Start date/time e.g. Thursday 25/06/2009 08:00
B1: End date/time e.g. Thursday 25/06/2009 18:00
C1: Duration of break e.g. 01:00 (1 hour)
D1: B1 - A1 - C1 in hours

Any ideas?