I was thinking about that (is the v4 version better than the GT version?) as I had the Netgear 834GT in mind.
The DG834GT is a different router. It uses the same BCM6348 chipset as the DG834Gv4, and is more expensive, which is why I suggested the latter. Their performance should be very similar. But you may have trouble finding a DG834Gv4, as it has been superseded by a v5 version using a Conexant chipset which probably doesn't have such good performance.
Do you think that it is the BT DSLAM that is maybe trying to automate things and it has set the SNRM too high? I'm thinking that it "sees" an artificial snr figure from my router which doesn't match with something else (maybe amount or type of errors either from my router or on it's database).
The 'culprit' is Dynamic Line Management (DLM) and it responds to what it sees as instability by increasing the target noise margin to give more room for changes in noise level, and so make the connection more stable. This is what must have happened in your case - there must have been a period of instability which resulted in the target noise margin getting bumped up to the maximum 15 dB. Unfortunately the instability could have been in part the result of your own tweaking, with the consequent re-syncs.
If I get the Netgear router and DGTeam software and configure it and save to give me an IP Profile of say 1500 does that mean that if the DSLAM decides to "retrain" with my router and wants to drop the sync speed/raise the snr to what it "wants" the router will force the snr down to the saved figure automatically so that the IP Profile won't drop to 750 which is what is happening to me at the moment?
If your target noise margin as set by DLM is already at 15 dB it can't go any higher. You can tweak this down to about 9 dB using any of the tweaking methods, and with the DGTeam firmware this tweak will stick, so DLM can't push it back up again.