So, this problem has now gone.
I can now ping any external host or IP address, resolving is quick and pages load normally.
In fact, quicker than they have ever been!
I purchased a new ER-X but have not changed it, so I can only assume that my Technical Support Ticket and Tweets to ISP Plusnet in the UK have been answered...
...although they maintain "nothing changed."
Yeah, right.
Sorry to have bothered you all here.
Regards,
Paully