Just in case this helps anyone in a similar situation.
Car was idleing very erratically upon start up, sometimes would try to die out when offering light foot pressure on the accelerator.
It proved to be a split on the inlet pipes just past the sensor. sucking unwanted air in; throwing everything out of sync.
Diagnosed by the local mechanic and promptly repaired.
£34.00 for two pipes, 1 hours labour and just over 80 pounds total bill.
Economical and easy fix, plus fuel economy is now back on track.