Not sure why but this User-Agent was blocking IIS7 (not my ASP.Net app but IIS7 itself :/)
To solve this, I tried blocking GBPlugin a few ways - URLScan, RequestFiltering and my own custom http module. However, eventually I worked out how to do it using the generic URL rewrite module:
Although I of course configured this using the IIS UI not going direct to the .config files - just felt safer on the live server.