Calculating hours that have passed from one date to another

Does anyone know how to work out how many hours have passed from a start date to an end date?

I.e Start Date/Time: 05/10/2011 10:00 End Date/Time: 06/10/11 12:00 (26:00 hours)

I tried subtracting one from the other with the date and time format (custom dd/mm/yyyy hh:mm) but it can never deal with anything over 24 hours.

Sheet attached with an example highlighted. Column E is start date/time; column G is end date/time; Column I is, at the moment, simply G-E.

Many thanks and kind regards,

Zak