ADSL2+ problem

Hi all
Looks like I have to do battle with my ISP (coms.com) so need to pick brains

My line has looked like this for ages and been perfectly happy-
Attn 46.5 db
Target SNR 6
Connection speed around 5.5 to 6.2meg
permanently connected to test socket

The line would not connect a week or so ago for more than a few seconds (9/5/15 I think it was). This went on for a few hours then suddenly started working.
When it did my margin was still 6db but I was only getting 4 meg. Did a router restart, same thing but at least I was online.
Contacted ISP, their first action was to turn interleaving off after the usual please plug into test socket request etc. I told them I was still connecting at 4 meg.
I came home tonight to an email telling me "we have changed the profile of the circuit to an interleaved 9db SNR one. This may well be more suitable than the 6db fast path you were on previously as your circuit is 3.2km long."

Guess what, I am connecting even slower! Arrrgggh!

Any tips on what to say to them? Seems to be a fault or a change made somewhere to me. Openreach have been around a lot working on cabinets dong FTTC work

This is still ongoing, Coms are saying its a noise issue causing a slower connection.
I actually had this issue about 2 years ago, back then I monitored the noise margin with "routerstats" and found that something suddenly caused massive loss of margin/synch, then stopped allowing me to connect as normal. Whatever it was it suddenly went by itself.
This issue is not the same, I am connecting about 2 meg less than normal constantly rather than intermittantly and I am maintaing a steady 6db margin.
Might not sound much but the 2 meg loss means I'm struggling to stream iplayer etc.

ISP saying they will send a BT engineer but I will be liable for cost if there is no fault found (Im betting they wouldnt find anything)

I have a gut feeling its something to do with all the work thats been going on related to the installation of fibre, feasible?