My use of "hard-coded" was not correct. I meant "are you setting it in the router?". On Plusnet you should not do that. Plusnet static IP addresses are in fact dynamically assigned, but always set to the one held in your account.

Rather than a simple power-down, a factory reset of the router may be a good idea. (Reset button while it is still powered up).

In reply to a post by flippery:

Appears there have been problems with Avast Safe Zone and Windows 7.
Have you tried disabling Avast

Yes, Avast! is disabled

Thank you, everyone, for your helpful comments. All is now OK.

Bypassing the router (PPPoE direct into the BTO modem) was a success so I isolated every component and started again. Turning off allocated IP addressing in the router made no difference but removing the other router (the Technicolour TG582 supplied by PN, which had DHCP turned off and was being used as a wireless AP) caused the problem to go away.

So... back to using my old O2 Wireless Box IV as an AP (DHCP turned off) and all is well.

It seems that even though I configured the T582 as a dumb AP (DHCP off, IP address fixed to fall within the range allocated by the W9780) it rebelled!

Thanks again for your help.