Slowly working my way through the migration permutations for FTTC, so after 12 months on BT Infinity 1 am now with another provider (will keep name secret for a little while until I see what performance is like over time).
DLM seems to have reset and dropped from 27-28 Mbps to 22 to 23 Mbps and not looked properly but guess G.INP is off now and may come back in time. Using identical modem, just swapped router over for ISP supplied one using its Ethernet WAN port. Plan is to play dumb and leave kit to get on with it rather than keep fiddling.
Lost old connection at 9am and new one I noticed was up and running by 11am (may have been earlier but busy using my other line), run two FTTC connections as work from home most of the time.
On the old BT Infinity 1 service was fine, never had a fault so never talked to customer service. Migration away all the usual letters and now to keep an eye on direct debit and final bills to make sure everything balances out.