I'm profiled on ADSL1, even though the line is physically connected to the ADSL2+ equipment in the exchange.

I think the outage was caused by underground works in the area. It does not explain why my line got fixated on 2MB, but I have a theory.

The line went hard down for around 48 hours. After that when it came back up there were a boatload of errors on the line, resulting in it constantly re-trying lower and lower speeds. This took a full 48 hours to settle down. During this time the BT kit was trying all of it's parameters to lock in a reliable speed, which was around 1.7 to 2Mb.

When the error conditions went away however, the line did not re-train back up to the previously achieved speeds, and needed human intervention on the 'profiler' (I dont know the term). The SNR was still very high though, and even though the line was totally clean and had a good signal was always not going to give the abolute best result. By the tech changing it and locking it in at 6dB at the exchange, it will do this.

Current stats from modem :
ADSL
Type Interleave Path
Status SHOWTIME
Downstream Upstream
Data rate(Kbps) 8128Kb / 832Kb
Noise Margin 7 / 11
Output power(dBm) 121 / 199
Attenuation(dB) 34 / 20

The SNR was still very high though, and even though the line was totally clean and had a good signal was always not going to give the abolute best result. By the tech changing it and locking it in at 6dB at the exchange, it will do this.

The ISP can trigger a complete restart of the 10-day initial period, with everything set to defaults. That is what I would expect them to have done.

In the past there was, and possibly still is, a facility simply to reset the noise margin. I'm not sure ISPs can still do that, as it had to be done by a request to BT Wholesale, not automated. I have a feeling this complete reset replaced it, as that is automated.