How can I resolve an issue with BOSS Therm BPS242RF failing?

Just wondering if you have ever seen an issue like this:

Heating setup: Boiler downstairs in kitchen, hot water storage tank in airing cupboard upstairs. One feed to radiators in main house and another feed to hot water underfloor heating in the conservatory. There is an electronic timer unit for the heating. Two remote control thermostats for the heating (BOSS Therm BPS242RF) . Three Honeywell valves (one for hot water and one each for heating systems).

Status: The hot water heating comes on as usual and works fine. The radiators and underfloor heating are not working correctly.

Symptoms:

- Sometimes the radiators in the house do heat up, either when they are supposed to on timer and also sometimes when I override on the remote unit. So sometimes it randomly does work, but mostly not.
- The underfloor heating never appears to work.
- The underfloor heating thermostat receiver (in the airing cupboard) often flashes with a red light, which I don’t remember ever seeing before, though don’t look very often in normal circumstances. The main house radiator thermostat receiver also flashes with a red light but less frequently.
- If I switch on the heating override, and manually move the heating system valve levers to open, then the radiators in the house do warm up straight away. I am not sure about the underfloor heating because it’s difficult to tell for hours… and the floor heats up from the sun after a while and gets too hot in there anyway. The red light on the underfloor heating thermostat receiver still shows up in this event.
- Whenever I switch the remote thermostat senders into override (ie on) the receivers in the airing cupboard always respond as expected, but you never here the valves whirr or the boiler firing up. I say never, it has randomly worked once in a while.

Tried so far:

- Replacing batteries in the thermostat remote senders.
- Making sure the desired heat setting is well above the room temp whenever testing.
- Resetting the thermostat receivers and re-establishing connection to the senders. Always works as expected.

It seems like the thermostat receivers are not telling the system what to do. I assume the red lights appearing on the receivers (mainly on the underfloor but occasionally on both) might mean something, but would seem highly odd for them to have both developed a fault at the same time.

I have tried troubleshooting on google, but the main advice is to reset the thermostat units – which I have done more than once.