Hi
I now believe that this checkout page not working error is due to the same issues that have just made the "contact.php" page non-functional also.
I believe that one or another helper webassist script is the cause, because as soon as I went back, out of curiosity, to test the contact page that had not been working, it, also, is now not working, and the only things that have changed is me having recently re-installed all my extensions due to having purchased a new laptop.
If that is correct, then I shall re-download every script in sight so all those on my local computer are replaced with your on-server corrections.
Thank you for your kindness.
KAB