So the problem continued, stair lights coming on knocking out the hub and the internet being dire - the Ofcom speed check service noting download as low as 2.4 Mbps, Upload 4.3Mbps.
Engineer no.2 reckoned the filter on the new socket should have stopped the hub dropping out however he surmised that possibly the lights going on might be causing a problem at the box in the street and said he'd renew the fibre connections there. Hub no longer drops out, internet back to normal (Ofcom check shows download 58.0/upload 18.1).
Result!
