Exhaust seal replaced, no change.
Original sensor has Bosch part no, different from replacement.
Refitted original, functions but takes seven minutes to change voltage when hot after immediate restart, sets fault code for slow response.
Fit a new sensor with matching part number and repeat test, this one reacts after 12secs, does not read continuously lean.

Conclusions.
Pay more attention to part numbers. We relied on Factors reg lookup catalogue, Glyn's suggestion was correct number.
Spend more time confirming nature of fault and rectification.