Thanks for your help everyone!
Yesterday I disconnected all of the extensions and connected each one up one at a time, taking a note of the stats each time.
With no extensions added here's what I got:
Master socket
Data Rate(Kbps) 4128 832
Noise margin (dB) 15.1 10.0
Output power (dBm) 11.9 19.7
Attenuation (dB) 46.0 28.0
Here's the best extension:
Data Rate(Kbps) 2720 832
Noise margin (dB) 15.2 10.0
Output power (dBm) 11.9 18.3
Attenuation (dB) 46.0 28.0
And the worst:
Data Rate(Kbps) 2048 832
Noise margin (dB) 15.3 10.0
Output power (dBm) 11.9 17.3
Attenuation (dB) 47.0 28.0
All of the extensions are round 6-way cables and I've only connected up the 2 & 5 wires each time. Are these figures what I should expect to see or is my cabling shot to bits?
I've connected the router to the best extension (wired to the back of the front plate) and Sky / DECT to the second best (connected via the front socket) and left the rest off as I don't use them.
I've got a Belkin F5D7633UK4 FW:UK_1.00.25 so I've been able to tweak the SNR down to 9.5 giving me a rate of just over 3000Kbps. It's now on an decent shielded cable which should have helped. And I've just ordered a Clarity faceplate so I can make a neat job of the cables and get rid of the (passive) filters.
Cheers!!