All is good
You were right - the code is good. Tonight I created a parallel cart (so to speak) by appending a 1 to all of the relevant files (e.g. cart1.asp) and changing the relevant links in the code. I uploaded it to the server and was able to checkout using this 'parallel cart' without issue.
So the error that I experienced must have been caused by Firefox using old code from its cache.
Tomorrow, I will upload the new files as the 'live' cart and expect all should go well.
Thank you for your help.