As optimistically anticipated, the downstream SNR has dropped to 3.0 and the performance test download speed increased to 3.6 Mbps. The download IP profile has increased to 3.69 Mbps.
I believe everything is now optimum given our EO line and the distance to the exchange. The speeds are consistent with the best obtained with other suppliers.
The problem was clearly being set to ADSL1 rather than ADSL2+ and I've found an article that states "we understand that BT may provide ADSL connections to new BT Broadband customers in ADSL2+ enabled exchanges if predicted download speeds are below 8 Mbps"... looks like this has been the case.
We've recently had a brand new Huawei All In One cabinet built on the pavement immediately outside our house. The pavement was also dug up along to the old PO manhole where I believe our EO lines run. 90% of the premises in our postcodes appear to have EO lines so hopefully the cabinet has been built to address this and provide fibre but I haven't been able to find any further information.
Thanks for everyone's help, I'll put the faceplate back on and mark the thread as fixed.
