Gave BT another call but they still wouldn't budge. So I've gone with TalkTalk; £26 pm after cash back for 76/20, no traffic shaping, no activation fee and fixed price for contract.
Plusnet worked out at £28 pm after cash back, once you factor in the activation fee and extra charge for caller ID. I read a few reports of people having slow downs from the traffic prioritisation. Which put me off a little.