OK so time for an update.
The HUB stayed connected pretty stably for a month or two, only rebooting once every 14 days which apparently is a normal thing for these? Until last weekend, where we dropped down another band to 30mbps. I'd observed the average SNR dropping down from around 8 down to into the 6's before the banding drop. We were one of the first on fibre on our estate, so maybe the effects of crosstalk from new sign ups?
Our current stats:
Data rate: 9.36 mbps / 30.00 mbps
Maximum data rate: 9359 / 38103
Noise Margin: 5.1 / 9.1 (downlink stays around 8.9-10 atm)
Profile speed (from the bt wholesale speed checker): 27.76 / 10
Speed test results: 27.12 / 9.1
Ping: consistently about 15ms on speed tests
Adsl checker results (I find it interesting the upstream has always exceeded these):
Quiet line test either in the test socket or main socket (we have no extensions anyway) has no pops or crackles, just a very quiet hum that increases as my hand gets close to/tightens around the coiled handset wire which I think is more a case of poor shielding/components than line noise? Wonder if you can get any line testing devices rather than just using a phone?
I have a HG612 coming from ebay that I couldn't resist at £12.98 delivered, but I'm going to be a bit hesitant about installing as from what I've read if it causes us to drop another band, DLM recovering it will probably never happen.