Thanks a lot everyone. I had Jeff's article marked down as something that would probably benefit this, but to be honest I haven't got around to it yet (guess what I'll be reading tonight before going to sleep? :-P)

As for the suggestions, immediately I've added a tally table with the changes Wayne suggested. I then intend to do some testing on my real data to see the performance differences between the others.

Celko - I know we have a TIME data type, hence why my original solution was using an on the fly table variable with times set to a 15 minute variable. Thanks anyway, but I suspect the number table approach will work better for my requirements since I need to be able to change the interval.