I had a similar experience to M Mouse with Pipex. They also ended up with TalkTalk, the performance dropped and in the end I broke the contract on the grounds of them not providing the required service
They threatened legal action for the unpaid bills. I'd cancelled the direct debits, and in theory owed them two months rental, but two months of substandard service - but they never came after me for the cash. I'd made a point of complaining on the online forum every day for weeks about the service, stating the speed, so they didn't really have any chance of enforcing payment. They would have been laughed out of court
I switched to Zen, guess what? End of problems. Back up to speed within a week