Woah, info overload!
Cheers guys. Some of it went clear over my head. However it has been a great help.
After due consideration I am:
a) Buying some different chemicals in order to rule chemistry out.
b) Putting a B&W film through the Contax (which I know I can develop) to check that the exposures are still okay.
As for the scanning: The duff film has scanned the same on two different scanners (which have produced good results in the past) with the same results, so I'm inclined to think they're okay.
I''l keep you posted with my findings.