5
that's great thanks Ghia!
the problem was a strange one.
If a user clicked on the 'register to post' in the newbb module they would come to this page and the url of that page includes the redirect, so like this:
http://www.domain.com/user.php?xoops_redirect=/modules/newbb/viewtopic.php?topic_id=883
If someone needed to register, for some strange reason the register link was
?xoops_redirect=/studyintheuk/newbb/viewtopic.php?topic_id=883/register.php
which obviously is a nonsense link.
Thankfully changing the link (in the file you gave me) like this:
"register.php"
to
"/register.php"
did the trick