Thanks for the suggestions so far.  I've tried them but to no avail!

I've installed all of the hotfixes related to the error and even tried SP2 CTP.

I now have a...

Thanks Markus but still no joy?

Very infuriating this one.

Ok, I sussed it on my own.  It turns out that you can't use a server name with underscores in it - a known feature it seems.  If anyone else has...

I found this on MSDN....

The report server supports several multi-stream formats that can be used to render a report. These formats include HTML3.2 and HTML4.0. MHTML, PDF, and Excel formats...

No ISAPI filters installed on the system, but thanks for the suggestion. 

It looks like it may be a permissions problem somewhere, but as I said you can render the report...