I have been with Nildram since 2003. They were taken over by Tiscali, then Talktalk and now Opal. My service had, until recently been ultra reliable up to 8Mbps contract giving a usual 5+ Mbps. One outage in all the time I have had the connection, until recently.
In the past three weeks I have been lucky to get 0.5 Mbps. Opal are utterly, utterly useless. I have had two 24 hr. outages in the past month since Opal took over and gave up on technical support on both occasions having spent over an hour trying to get through each time. Their ring back service rang me back once and put me on hold! Opal say the line is faulty. BT, who I have spoken to several times, have been very helpful and run various tests and claim the line is fine.
After two weeks of being passed backwards and forwards with no resolution and an abysmal connection I have given up and asked Opal for my MAC code in order to leave. They then tried to persuade me to have my telephone service with them because then they would be able to solve the problem! Been waiting for my code for three days now.
I am going to go with BT, upgrading to their new fibre optic (alleged) 40Mbps service. My reasoning is that with one service provider to deal with for line and BB service I can at least just have one company to complain to.
Sorry for the diatribe but it might be the path to tread in your case, go with BT alone.