I don't know whether or not this might be the right thing for you, but in the past Andrews and Arnold have had a we’ll fix your line scheme and they have attracted a reputation for taking wholesale service providers to task and fighting for the customer. Anyway, might be at least worth a casual chat with them. They are on IRC for example.
Agreed. If I were you, broadstairs, I'd contact Andrews and Arnold or at least go on IRC and see what them or their customers have to say. I'm betting they'll say "move to us and we'll sort things out for you". They have a strong reputation for kicking Openreach's ... yeah, and usually get a good outcome in the end. If you end up going via TTB instead of BTw then they also seem to be pretty on top of things if AAISP want TTB to be. Both backhauls are now part of the DLM trial as far as I know, previously TTB wasn't which was why it took a while before my line could get a DLM reset performed (banded).
I'm also curious to know what modem you're using and whether you tried a different chipset when you were fastpath? For example, I'm on an ECI DSLAM (unfortunately) and I've found that the Infineon/Lantiq chipset in a modem works better on my line compared to using a Broadcom chipset. I get around the same sync rate as the Broadcom on my current two lines (but all lines work differently), if not better (by using a negative DS SNRM target offset). I get fewer error seconds than a modem using a Broadcom chipset. Latency is also a millisecond or two lower than Broadcom.