Tried it again and now it's connected.
It's reporting: Upload Download
Actual Data Rate 20000 79984 Kb/s
Attainable Rate 26527 89037 Kbps
And the online status shows as:
VDSL Information (VDSL Firmware Version: 1412f0 )
Profile State UP Speed Down Speed SNR Upstream SNR Downstream
17a Showtime 20000 (Kbps) 79984 (Kbps) 99 (0.1dB) 94 (0.1dB)
TBB Speed gives me this:
Speedtest gives me this:
is reporting 71.6/8.87 which is inline with what it was reporting after my first abortive attempt and I'd put the ECI modem back in place. Pre my fiddling, the BT checker was reporting 72.15/15.34
I think I'll leave it as is for a few days and see what it looks like.