Update on this issue:
A reboot appears to have solved the problem, allowing me to activate the ssl-proxy-list after completing the urlrewrite configuration. Also testing has shown that the re-write works as expected.
Don't know why this should be the case but the testers had indicated that the CSS had stopped "working". After checking all the config (as I had made some changes to try and isolate my proxy-list issue) I decided to reboot (test environment) and this happily allowed testers to re-connect and fixed the SSL proxy-list problem.
I would still be interested to learn what it might be.
Thanks,
Brendan
