Checked this out: there is about a 7 degree difference between instrument and ECU readings, but ECU is getting the higher reading. Even so, my gut feel is this is Febi tolerances rather than failure of a new part.
On the other hand, with the revs wedged at 1500 or so, almost straight away the top hose started to warm up. With the car sitting on the drive with raised revs, the temp gauge eventually got to 70 on a sender reading of 65. Other than a pound up Reigate Hill, that's the only time I've seen it move. I have no doubt had I driven up the road on this chilly evening the gauge (and temp) would have dropped again.
I'm calling either thermostat or faux-pas on the installation, ie coolant flowing around outside of the 'stat. Not end of the world, I need to jack car back up to change the gear oil (dreadful gear shift so will do the quadruple set of lube tower (again), lube cables, strip / clean / silicon lube shifter mech, plus that oil. Worked on Cobalt II which was nearly as bad but now as good as it can probably be.
Assuming install was OK I'll introduce thermostat to saucepan and digital temp reader, mainly to get refund plus costs if faulty.