Well after leaving things alone and letting dslam do it's business the single thread speed as I excepted returned to normal however the connection isn't stable so guess I will need to call them again.
Had BT out again today as the line was busy resyncing and speeds down to 43mbps on the original line.
The engineer noticed a lot of crc errors despite interleave being on so he went to the junction box on the pole and plugged in there and no errors.
So he replaced part of the line as he didn't want to run right round the house. The damage suspected damage was round the area of the support and when he replaced there was a sharp kink at that point. The first I noticed was that it synced straight away. Ping times dropped from 22ms to 11ms the original line went back up 55mbps even better the 2nd line was now showing 72mbps and full upload speed again.
I'll keep monitoring for the next few days and see how things go.