A bit of a long shot but if you have a spare router connect it up and see if the connection drops as often/or at all compared to the one in use.

Snap, GMTA

You've obviously got a faster BB connection than me

Well, afraid to say that after trying at least three different routers over a couple of weeks, the Sync speed hasn't improved at all. With one router (a slightly older Netgear that I borrowed briefly) it even went down a little more! [Not what I wanted to happen!]
To my mind this all just goes to re-enforce the feeling that some engineer has done something in the local exchange which caused the Sync-rate on my service to be reduced.
When the two BT engineers who came out here last (probably 2 years ago now) when my service was down in the 1Mb/s region, they were seeing syncs at around 6.5MHz when they left, and that then slowly rose (through DLM I presume) until I was getting a fairly steady 7.5Mb/s - until that fateful Saturday morning.
Now I can't get anyone to even look at the service since I'm "receiving something which is above the minimum stated in the contract to supply."
To me, this is symptomatic of the feelings that BTO simply aren't concerned to get the best out of their systems - 'Good enough' is now acceptable. Bah!!