Hi Barry
1/. Towlerg: "Why not use a logic analyser?" - simply because I don't have one. I also need to replicate this test in other parts of the world so I don't think purchasing multiple logic analysers is a cost effective approach to solving the problem.
You can buy a Saleae for 119Euros. Accepted that you'll need an old laptop for display. It'll store billions of samples @ 24Mhz, thats at least a decade better that Darrel is suggesting, and he writes great code. How many samples before the PIC runs out of memory?

Anyways, good luck with the project.

George