Solved...so not an emergency anymore!

This is an edited response from my web host...it seems XOOPS is recognised as being superb even by web hosting firms

"We made some slight changes yesterday, and set http://www.f3.org.uk as a symlink to point to f3.org.uk

Normally this would work fine, but it seems that the security routines in XOOPS are well-written and recognise a symlink from the true path.

If you set the server path in the configs to /content/StartupHostPlus/somevalues/f3.org.uk (remove the www. before f3.org.uk), then it should work correctly again.
"

And it did! I changed the values as suggested, and also the XOOPS_ROOT_PATH check back to 1 in mainfile.php so that all security checks are carried out again. It's strange that my phpinfo.php file not detail the physical path as being different, although I wonder if this was due to caching on the server or something.

My sincerest of thanks again to Skalpa for providing a workaround and Herko for being very much correct in the cause of the problem.

Cheers, and rock on!!