Plusnet are being quite helpful - they've had me doing various line tests and sent me another router to test, but nothing seems to help. It's not my PC because none of the PCs in the house can access it via the broadband connection, but can via 3G.
If anyone is techy-minded, I have done a tracert which shows it stopping at the forum's hosting company, and yet it is linked to our broadband somehow. It's very curious.
If anyone else has had trouble with Plusnet, or alternatively if there are any techy wizards out there who can suggest a possible cause (or better still, a solution), please get in touch! I can still read via Proxify.
Many thanks