The ISP is BT in this case - the livechat person was sure that it was going to be a wholesale engineer rather than the same engineer type they sent last time. The tech on friday did say he couldn't see any kind of cap or porting issues though... I'm not holding out for much but we'll see!
I just tested the line this morning with and without the faceplate on the new mk4 filtered socket and it seems about 1-2mb faster without the faceplate on and using an adsl filter I had lying around.
If I get the new tech to disconnect the wiring from the master socket to the other sockets in the house would it be expected to get the same speed as the test socket? I dont use the extensions at all so will probably end up dragging cat6 through them in the future.
Thanks! I'll report back tomorrow to try and conclude a happy story!