Suffering my usual aversion to physical reality and obtuse/opaque systems, I reluctantly went upstairs to the airing cupboard and examined the Siemens RCR10/433 wireless 'base station'. All the lights were out. I prodded the 'Set' button ('Do Not Touch This Button') and then, when nothing much happened, pressed the even scarier 'Reset' button. Then I turned all the power off at the socket and turned it back on again.
This morning I changed the batteries on the Siemens RDJ10RF remote unit while Clare suggested we move blankets away from the boiler and hot water tank to 'prevent overheating'.
As magical thinking failed us, I recalled Adrian's advice of the previous evening and googled 'How can I reset Siemens RCR10/433'. And here was the advice.
If the Siemens thermostat remote unit (RDJ10RF) loses connection to the base station (RCR10/433) next to the boiler (possible symptom: the base station green light goes out).
1. Make sure the thermostat remote unit has its vertical switch set to 'run' and the horizontal switch set to 'auto'.
2. Loosen the screw holding the thermostat backing plate (situated underneath, just loosen the screw don't take it all the way out). Take the unit to the boiler.
3. On the wall unit/receiving unit next to your boiler there are two buttons 'Set' and 'Reset'. press and hold the reset button for 4 secs until orange light starts to flash.
4. Then press the 'Set' button on the same unit (the light should flash again).
5. Press the 'Learn button on the back of the thermostat unit - try to be a good meter away from the wall unit /boiler. The lights should flash and then turn green.
6. Press the 'test' button on the back of the thermostat. This sends a test signal to the boiler . The green light should come on again. When this has happened press the test button again to switch off the test signal.
This should now work.
And miraculously, it did. There was, however, more information on the Internet from those people for whom this procedure had not worked.
"I too have a malfunctioning RCR10/433 which responds to a sharp tap. I've done a bit of investigation but not fixed it yet, thought I'd share what I've found so far.I went upstairs armed with a hammer, but was saved by Clare's ecstatic call of "You're a genius!"
There's a click when the orange light comes on, but the relay contacts don't switch over until given a tap (checked with a multimeter).
There are two obvious possibilities here - duff relay or failing power supply capacitor. The click from the relay suggests the former, but I need to check the voltage applied across the coil to confirm ... ."
The rewards of being in trade, even as a complete amateur.
Update September 14th 2015. The central heating has failed to come again today. I did 'all the above' but this time it failed to have any effect. I even tried (gently!) tapping the base station upstairs with the hammer. I can only assume that there is a fault in the room-remote or boiler-end control unit. We are in process of calling in an engineer to take a look ...
... and five minutes later, after calling an engineer, I go upstairs and tap the wretched base station with the hammer again (perhaps a little harder). And mirabile dictu, the heating turns on ...
Update September 23rd 2015. A qualified central heating engineer has now visited and checked everything. He informs me that the controllers are fine; the problem lies with the (rather old) three-way valve which routes hot water between the boiler and the hot water tank and/or the central heating system. Apparently it's sticking and not always responding - he will order a replacement.
I am put in my place (along with the Internet).
Update October 14th 2015. The engineer has been busy and has not yet come. In his absence the system has continued working rather erratically. I checked the installation guide for the three-way valve (The Honeywell V4073A) - it states that the operating temperature must be less than 52 deg C. I wondered if it was overheating so I cleared clutter around it and left the airing cupboard doors open. Since then, cross your fingers, the central heating has been working OK.
Update October 22nd 2015. The system continues to behave erratically. The Honeywell V4073A three-way valve has hot water in all three of its attached pipes when the CH is set to 'on'. This indicates the valve itself is working; it's just that hot water is not being pumped around the house. The engineer is tied up with a house-construction contract and is not available until the beginning of November - but he has given me the number of his former boss to try in case we need action sooner. I'm on the case.
Update November 4th 2015. The central heating engineer, Ian Hosegood, came today. 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 above. Ian switched it out for the Honeywell CM927 wireless programmable room thermostat and upstairs relay box.
Finally, the central heating is working again.