OK so an update on my complaint letter.
Got a call today from head office apologising for her colleague not calling me back weeks ago when promised, apparently he's been out of office...
Anyway, she has confirmed that £54 has been credited to my account for loss of services (latency) since Decemnber, so I questioned where the credit was for the high utilisation issues that still ongoing in this area (tbf it's better now, but still there, the BQM isn't flat at peak times, for example).
She confirmed that things have improved but the area is still affected by utilisation, and their new fix date for this is April 12th 2017. So in light of this, she added a further £24 credit to my account up to the 12th, and would call me back the following Monday to check if the utilisation issue has been sorted or not.
Obviously they have now closed off my complaint as the letter was specifically in relation to the SH3 latency issue, and that is now resolved with the SH2ac.
£78 credit for the whole ordeal, I'm somewhat happy with that, and I have a new SH2ac to keep me busy until SH4 comes out I guess and we are approaching Gigabit rollout.