Calculate response time in working hours between two dates

Hi - I am new to the forum and hopefully someone can give me a hand...I am trying (in vain so far) to calculate the response time to service calls, based on working hours.

Basic data I have is,-
Date call received (DD:MM:YY HH:MM)
Date call completed (DD:MM:YY HH:MM)

Working hours are from 08:00 to 17:00
Working days are all days, so 7 days a week 52 weeks a year

So, for example a call is logged on 06/06/2012 09:07 and completed on 06/06/2012 11:48 the answer is simply 02:41. Easy, because both start and end date are on the same day and in working hours.
But, if a call is logged on 06/06/2012 14:52 and completed on 07/06/2012 14:05, I can't for the life of me establish how to factor in working hours and come up with the right answer (which should be 08:13).

I have played around with Networkdays for ages but as we operate 7 days a week I'm not sure this applies??

Your help would be greatly appreciated.
Many thanks,
Andy