Someone alerted me to a Netscape problem when using this hack.
I seems that Netscape ignores the < BASE > tag.
Subsequently, any urls that do not start with a slash or a fully qualified domain will fail.
The solution is to go through all the showthread, forumhome and forumdisplay templates and prefix all relative HREF's with the $ bburl variable.
This means the hack isn't quite so 'quick and easy' after all
If someone knows why Netscape is doing this please let us know.