This has been a long saga - amazing how many hits this post has received (91 as of today).
The central heating engineer, Ian Hosegood, came this morning. It turns out the problem was indeed with the Siemens wireless base station (wall-mounted next to the boiler). It was showing the radio link to the living room thermostat as active, but the relay to the boiler/pump had not tripped. This is the bug in the device highlighted in the post I mentioned. Ian switched it out for the Honeywell CM927 wireless programmable room thermostat and upstairs relay box.
Finally, the central heating is working again.
In passing, Ian observed that I seemed to have a flat rear tyre (my car parked on the drive outside the front door). At the garage they found the screw causing a slow puncture - fixable; the other rear tyre was low-tread so had to be replaced. With the water pump failure last Friday, this has been an expensive few days.
Clare and myself visited my mother this afternoon - here's one I took with her while Clare was at the cafe getting us all hot drinks.