Ideal Logic+ combi and thermostat problem...

This is money saving in as much as: can I fix it myself?! Heads up: I know nothing about boilers, other than how to set the dials on my one, and how it responds to a thermostat! I'm also very logical, so have come to the following conclusions...

Have a Logic+ combi boiler, and a mechanical thermostat. The boiler definitely works to supply HW and CH, no issue there. However, the receiver on the boiler and the thermostat don't seem to be communicating anymore.

I've replaced thermostat batteries. Twice. I've re-set thermostat. I've switched boiler off and on and pressed (what I assume is) the receiver's re-set button. I've put thermostat nearer the boiler. Then re-set them both. Nothing. Nada. The boiler simply states it's in Winter mode (good) and that the thermostat is off (not good). Tried thermostat in the 'on' and 'timer' positions and neither message seems to get through.

Is it a case of simply replacing the thermostat? Or the receiver too? Would I have to get a plumber in for that? My plumber is booked for next Weds but just wondering if this is a simple thing. Not sure if I can buy a different thermostat and it will still talk to the current receiver? Not sure how they communicate.

(The reason I know the CH works - and the reason I think it's the thermostat and NOT the receiver - is because I had old batteries in the thermostat the other day, and the boiler seemed to go into Emergency mode where it runs for 4 mins, then does pump overrun for 9, and repeats continuously. The user guide suggests that it goes into emergency mode when it recognises the batteries in the thermostat are dead. Hence I know the heating does work, and it seems that the receiver recognised the problem. Having said that, the batteries weren't *that* dead, as the clock on the thermostat programmer still worked...so it's possible the boiler was in emergency operation mode for another reason.)

TIA if anyone has any suggestions.
KiKi