Quick update for anyone in a similar situation.
Bought HG612 Rev3 - unlocked. I checked the line stats, I'm no expert but I compared them with others on forums and the stats look 'good'.
Digging further into the cabinet looking on telecom tariffs and running a command on the telnet interface of the HG612 both confirm the cabinet is ECI.
I was getting variable speed test results and was concerned that all this disconnection / changing of modems had started to introduce some sort of line management. Looking further into the test results it's down to what server Ookla chooses - Newcastle always gives 37Mbps with a ping ~40ms. The two in Carlisle often give 26Mbps which I had misinterpreted to mean the line was slowing down rather than considering the traffic on that particular server. When I manually select Newcastle as a server the results are consistent. BT wholesale speed test still gives me 27Mbps.
Any thoughts on the following appreciated:
Should I be concerned about the discrepancy between BT wholesale test and the best Ookla result?
I've plugged the ECI modem back in and am using that at the moment. Has anyone experience of line speeds dropping when using the HG612 on an ECI cabinet?
My ping is ~40ms, is there any way to improve this?