Mr O - the 'splitter' or SSFP or I-Plate does, I believe, do a bit more than you think ie it filters the phone socket at the NTE5 (ie 'splits') as well as isolating wire 3.
Anyway, things have moved on - internal wiring checks out fine - no difference in errors or speeds connected or not so the next step is to try another filter and find a mate with a spare ADSL router to swap out BEFORE I call BT out to find it is my router, and pay them the odd £100 for the fun of it.
