Fully pumped system with no zone valves - how to add room thermostat?

Hi all, some help required please!
We've moved and inherited an oil-fired heating system with two pumps (CH and DHW) but no zone valves. There is a cylinder stat but no room stat, so we would like to install a wireless room stat. But I've not come across a two pump system before (although I guess with two pumps the zone valve is not needed).
The current programmer is a Sangamo Choice PR2, two channel, set to 'gravity' instead of fully pumped and using only t3 (HW-ON) and t4 (CH-ON).
I would like to replace it with a Honeywell Sundial RF2 Pack 2, but not sure whether to set it as gravity (C-plan?) as per existing Sangamo, or fully pumped.
I'm told the previous owner fitted a Hive as a simple swap for the Sangamo and this worked but CH would not come on without DHW, so I guess the same might apply if I just swap in the Sundial? I could live with that, or maybe add a simple C/O relay at a later stage?
Many thanks

Motorised valves have internal microswitches so that when the valve opens, the microswitch operates and switches on the boiler and pump.

As you just have two pumps, this 'microswitch' facility won't exist, so to get around it the system has probably been wired such that when the hot water is set to be 'on' the boiler and hot water pump will operate, and the central heating pump would then be wired to operate when the programmer turns the central heating 'on' however because of the absence of the microswitch it can't start the boiler, so the hot water which is controlling the boiler will need to be on as well.

If this is the case, you could probably still easily add a room thermostat to control the central heating pump, but the 'gravity' mode would have to stay to start the boiler.

If you wanted fully independent control, it could be possible to wire a relay to each of the pumps to provide the same switching function as the microswitch provides. Or have the system re-plumbed to incorporate motorised valves.

The microswitches also provide a boiler interlock, so that when room and hot water cylinder thermostats are satisfied the boiler is switched off too. With your current arrangement, it is likely that the boiler would remain 'on' and keep itself hot when the room or cylinder thermostats had switched off.

Hi Stem,
Thank you so much for taking the trouble to explain the probable setup. I'll replace the Sangamo with the Sundial set for gravity and live with the lack of independent control for the time being. If we actually get a summer next year and sparkies become available once more, I'll probably take the opportunity to have some relays installed. Re-plumbing probably would prove too disruptive, especially as we hope to migrate to ground-source heat pump in a few years if the grants become attractive enough. Again, many thanks for your time.

You are welcome. I'm glad you were able to decipher my rambling explanation.

Seemed clear enough to me! Perhaps you should offer your services to Honeywell, as they seem to limit their support to what you can read for yourself in the wiring instructions (if, that is, your system is the same as one of their plans ....)? Have a fine weekend and take good care!