Filler cap holding water...

[Borderbob]

Stripped the unit out today and, as Rixter suggests, the blockage seems to be where some welding has been done - no way will it clear. New unit fitted within half an hour without any problem at all. Thanks again for the advice everyone.

You are right, if its the same as mine you'll find it's welded up! Crap workmanship. See here for pics and how to fix it.
http://www.ukgser.com/forums/showthread.php?p=2808899&posted=1#post2808899