Ok guys, a quick update on the situation following BT Openreach visit yesterday afternoon.
The engineer, who coincidentally was the same one who installed my O2 line in February last year, did come and check everything for me. Some of you may remember a previous topic started by me concerning differences in 'loop loss' between my new O2 line and a line serviced at the time by Plusnet - this service is now non operational, although the NTE box is still present. As I live in a flat, there is a DP in a communal area, and after plugging a piece of test equipment (possible some kind of tone generator) into my NTE box, he took a look at the wiring inside the DP box. He immediately found a loose / disconnected wire which I think he referred to as HRDIS, and which he suspected could well be the source of my recent problems. Having sorted this out, he then went to the street cabinet, and found something else of concern, and from there onto the local exchange to do something else before coming back here to tidy everything up. He was here for a little over an hour and I did my best to provide adequate refreshments while he was here. He was a very nice guy, very helpful and informative, as I believe most BTO engineers are, at least in my experience.
During our conversation, we chatted about the forthcoming installation of FTTC at my local exchange (Weston-super-Mare), which according to him should be completed by the end of 2012. In his view, he said that based upon my current installation I might be fortunate enough to achieve up to around 36Mbps, possibly a little more - weehee !!!! He estimated that the distance to the street cabinet for my line is approximately 300 metres, although there is another closer cabinet which I estimate to be around 100 metres from my building. Both of these cabinets are quite old, and will be being replaced or whatever in the coming months.
A quick thank you to all those who have taken the time to respond to my request for information about 'Battery Contact Fault'. Foolishly I had been thinking that it related to a battery somewhere in the network, but I now know differently - so thanks.
I guess it makes sense to let my line run for a few days and just confirm that there are no more 'line bounces / speed reductions' before I approach O2 support to try and get my Target SNR reset to a more sensible level (currently at 15, I believe), and hopefully get back a large part of the speed that I have lost over this incident.
P.S. I have been monitoring the line with Routerstats since a couple of days after the initial problem of 12 September. It took me a while to set it up to work with the O2 Wireless Box V, but I got there eventually. I now have more stats and graphs that I can shake a stick at.