I have been getting the issue again intermittently pretty much all afternoon.
Strangely, sometimes it is only certain websites that redirect me to the BT page, eg: www.consolerepairs.co.uk
I have stayed connected to Plusnet throughout. Just tried disconnecting and re-connecting to Plusnet and as it stands I still cannot get to the above website.
Have written to Plusnet again, and am awaiting a response.