OK, secret to good diagnostic procedure, take a day off, go fishing, appreciate the landscape and come back in this morning with a fresh approach!
Stripped the heater duct out so I could get a closer look and double check my previous results.
Pin 1 at the sensor should be the + feed of 5v and that is at 600mv (so straight away there's the problem) Checked the voltage at the ECU and it's 5v, plumbed in new cable and hay presto engine runs.
Thanks to all that contributed and especially to Pete for the wiring diagram.

P.S. Fitted genuine sensor from our Ford supplier and it was £10.00 cheaper than the Lucas "branded" item
Mark