I can't see any obvious pattern and I know one of the engineers at BTw was also looking for a pattern but couldn't see anything obvious (unfortunately he's now disappeared off the case as he was the one who initially picked up on the errors).
I have removed the ringwire but that didn't do anything. The BT Vision guy did remove a cable between the vision box and socket (he said that it wasn't needed, although the manual says it is
). That did have an immediate effect cos at that time the margin was at 18 and straight after dropped to 7.5.
I'll have another look and see if I can see a pattern and I'll go thru the list you provided to see if there is anything else I can do at this end. I must admit the BT Vision guy did fit a filtered plate for me but had to take it off cos Vision and Hub wouldn't work (both these are connected to an extension BT put in when vision was first installed).
Thanks for your help again and at least i can now get onto BT and tell them that sorting out these errors may actually resolve in me getting better and more stable speeds.