Ah but my present concern is line stability rather than speed. Would LLU elp that issue if IDNet don't come good?
Depends what the problem is; if its physical on the copper (or aluminium) between you and the exchange and some rubbish connections, then the problem will follow you to any ISP.
If the problem is caused by BTwholesale's DSLAMs, then changing to another BTwholesale (20/21CN) using ISP would probably make no difference.
LLU providers install their own DSLAMs into the exchange building and pay OpenReach to connect your cable to their equipment instead of BTwholesale's. Then they can offer products and settings that BTwholesale don't make available to their customers. Generally this results in more usage for same money - or alternatively really low monthly fees (e.g. TalkTalk) - because the ISP doesn't have to pay the BTwholesale charges.
Also an LLU ISP can set technical parameters directly (interleave, target SNR margin) without having to go through BTw systems.
BTW I've just checked SamKnows and O2 and Be* are listed together. Did one buy the other out?
O2 is owned by Telefonica, and Telefonica bought BE. We understand the O2 LLU products are run by the BE technical team, on the same network but the traffic is not joined. O2's products now have traffic shaping for the money - BE's products are higher cost but no traffic shaping.
pro - on THFB - sync about 17.2mbps - BQM