Eventually I reported this to my ISP, and BT re-wired and my speed jumped:
Initially syncing at 79428, then after a week a resync to 79996 followed by a series of resyncs in fairly quick succession to 70480 (figures courtesy of the Zen Portal, thanks guys!)
I had bought an HG612 previously from ebay to find out what was going on - but never deployed it. I have now unlocked it and installed it as a replacement for the ECI.
I am now syncing at 69636
A few questions now!
How does the Huawei perform vs an ECI - am I doing myself no favours by swapping?
Will my sync speed ever creep back up or is it only on a donward spiral?