What happened was the following:
1. Tried to trace internal wiring from entry point, but internal ducting made that impossible
2. Made a guess on the socket, disconnected one side of wiring
3. Found out which socket was first in the daisy chain wiring
4. Found my old SAR-600E got 3.7Meg sync
5. Attached the extensions - sync dropped again
6. Proceeded to try and figure out the sequence for the sockets in the house
7. Fixed the socket with one leg disconnected - sync improved, some odd noise on line
8. Noise went away after playing with another socket
In terms of wiring no real changes, just the IDC's had been remade, and confirmed no ring wire connected.
The 4167 sync appears to be with a 12dB target noise margin, so hopefully once with Vivacciti either it will reset to the 6dB margin, or it can easily be requested. This is likely to take sync to 5 or 5.5Meg, which is about right for the 47dB attenuation.
Moral is that never accept a poor speed until you've made sure the homes wiring is not causing some of the problem.