Based on past experience, I'd say you're right re the Thermostat. It appears to be working as the boiler is firing, i.e. it is calling for heat.
I would first check the automatic zone valves, as they do fail (I've replaced a few over the years).
Normally, the thermostat, when it calls for heat, sends power to the zone valve, and this opens. On opening, it operates an internal solenoid switch, which sends power to the boiler (you might hear this switch operating if you are close to the valve). So the fact that the boiler is firing would suggest that the zone valve is working, but the fact that there is no heat going to the rads implies that there is a fault. It is very unlikely that the actual valve is faulty (the physical valve that is opened by the electric motor in the zone valve - the motor sits on the head of the valve, i.e. it is one unit).
First thing I would do is locate the two zone valves. Then manually open the zone 1 valve. They normally have a manual slide that can be pulled across (and is normally pretty hard to move) and locked into position. Do this on the Zone 1 valve and confirm that the boiler is firing. Also confirm that the pipe on the downward side of the valve is getting hot, i.e. there is water flowing through the valve.
The valves sometimes have a light to show that they are operating, and normally the head unit (the electrical part) can be unclipped from the valve (which sits in the water flow). Again, seeing what is actually happening can give you an indication as to if a part is faulty.
I think (but could be wrong - maybe a plumber can chime in) that the boiler controls the actual water pump (which should be located near the boiler). Make sure that the pump is working (either by listening or feeling for vibrations with your hand). If the pump is working when zone 2 is on, but not zone 1, then this could be a problem. Unlikely, but you'd never know.
At this stage, you are gathering information on what parts are working an what aren't, and, based on what you have said, the parts that are possible culprits are the zone valves, both header unit and valve, and the pump.
Adopting a methodical approach should help in isolating the part that is actually faulty.