Hi James,
The below code should do the trick in a task report:
displayname=Proj Start - Task Comp
textmode=true
valueexpression=WEEKDAYDIFF({project}.{actualStartDate},{actualCompletionDate})
valueformat=HTML
This code will work out the number of weekdays between the project actual start date and the task actual completion date. If you don't want to use weekdays, you could simply replace "WEEKDAYDIFF" with "DATEDIFF" and weekends would be included in the calculation.
Hope that helps!
Best Regards,
Rich.