I have had a Honeywell CM927 Wireless Thermostat operating the heating
boiler without a problem for the last ~8 years.

This morning, after trying to turn the heating on - it is quite cold
outside, so I thought it a good time to see that everything works - the
heating would not start...

I set the target temperature on the thermostat to very high (25c, room
temp is 20c). The green light on the relay box comes on as it should,
but then turns itself off within a few seconds/minutes for no apparent
reason. Pressing the Manual Override button on the relay box causes it
to turn on, but again, it turn itself off almost immediately.

On closer inspection it appears that:

- The boiler is in order
- The thermostat itself work as it should: communicates with the relay
box, sends a signal to turn the heating on, etc

I therefore suspect the BDR91 relay box is the culprit.

Am I on the right track?