Try A&A. Pricey but decent support and very effective at dealing with openreach
We'll fix your line even if you are with another ISP!
If you are migrating your service to us, even though you know you have a problem with your line, we'll take on the fault. We'll tackle the problem and get it fixed within one month. If we don't then you can migrate away and owe us nothing for your migration to us and your service charges for that month. Details.