An update - having been '
assured' by BT that my line and the exchange line was 'top notch'
, BT and their contractors have been digging up junction pits in my road for around 3 weeks, with crossed lines, no lines, crackly lines etc. They 'discovered' some EXTREMELY old cabling, some running UNDER the new junction pits and with old lead joints. Two houses in the road now have new feed lines.
I am pleased to report that my connection has now been virtually faultess for a few weeks.
Lesson: don't give up - they 'flannel'