I am so glad I stuck with Plusnet and didn't give in to the temptation 
It's not really a problem. The 'service' that they're providing is so far adrift from the one they promised (I did get some written assurances before switching) that they'll have no option but to agree to accept termination of the contract without penalty. That will enable me to go back to my original ISP as a new customer, with the usual new customer pricing.