Just thought I'd report back since I hadn't posted an update for awhile.
Once the IP profile had sorted itself out I decided to switch out the HH1 for the HH2 I'd brought back from Coventry. It seems to be a lot more stable than the HH1 on first impressions.
The phone sockets all worked after plugging the test socket back in, so that's all good! And we seem to get a clearer sound on the phones now when talking to people, atleast I seem to think so! So thank you all for your help with the rewiring I learnt an aweful lot, and am very thankful for the knowledge that you've passed on to me.
Since the HH2 has been plugged in, the ping has remained constant at ~27ms during offpeak, and ~43ms during peak times. The SNR sits at between 4.3dB and 6dB during the day, and between 5.3dB and 6.8dB during the evening. The router's synced at 7616Kbps on IP profile 6500Kbps, and we were achieving a max of ~6470Kpbs which was expected with a line attentuation of 27.0dB with interleaving turned on.
I have now had interleaving turned off, thanks to BT_Care on this forum, and am happy to report that the line seems stable. I did need to restart the router myself, but since then it seems to be working flawlessly. Pings to UK servers are averaging an all time low of ~15ms! (And I've now got my beloved PS3 back so can't wait to get back to gaming, plus Starcraft II can be removed from its wrapping ), the router has synced at 8128Kbps happily, and I'm now just waiting for the IP profile to rise to 7150Kpbs and our line to max out hopefully at ~7meg.
All in all, it's a vast improvement from the 2-3meg we were receiving, dropping to almost 1meg during peak times.
Thanks once again to everyone who's helped!