Many thanks David.
Here’s a funny thing: Sunday morning I dedicated to checking every single fuse in all locations and to identifying “likely” relays, then swapping and sacrificing known ‘good’ relays for those that switch and control relevant functions. Zip. Nothing.
Left the car alone for 3 hours and then went back to it: tried the key in the starter, and, yes, started.
Cannot be a fuse, so I suspect sticky / temperamental relay. Will deliver the car to a Coventry-based specialist and ask his advice.
Just tried it again: yes, good as gold; started on the key.