Would the motorized valves you have on your heating system be Honeywell also? I'm familiar with Honeywell V8043 series of motorized valves for hot water heating systems which are as common as a barking dog over here, but not with the 3-way models that seem to be commonly used in the UK.
I'm thinking that the reason why the plumbing contractors you spoke to suspected a thermostat problem is because Honeywell thermostats (and I expect all thermostats) have something called a "heat anticipator" in them, which is nothing more than a small electric heater that heats the bi-metallic spring inside the thermostat to fool the thermostat into shutting off the heat early.
The reason why you want to fool the thermostat into shutting off the heat early are several:
First off, on a hot water heating system the cast iron radiators are going to keep convecting heat into the room as long as the water in them is still hot, NOT just when the thermostat stops calling for heat. So, you want the thermostat to shut the boiler off earlier so as not to overshoot the thermostat set temperature too much.
Secondly, the radiators are located around the perimeter of the house, whereas the thermostat is centrally located. If you waited until the air temperature at the thermostat reached the set temperature, the average temperatue everywhere else in the house would be higher than that.
Typically, the way a heat anticipator works is by having a variable electric heating element situated right next to the bi-metallic spring and wired IN SERIES with the mercury switch so that the heating element only works when the thermostat is calling for heat.
As you may have already guessed, the heat anticipator setting also affects the boiler cycling rate. A very low heat anticipator setting will cause minimal heating of the bi-metallic spring, and a lot of overshoot of the thermostat temperature setting and a excessively long boiler cycling rate.
Conversely, a very high heat anticipator setting will cause rapid heating of the bi-metallic spring, thereby fooling the thermostat into thinking the whole room is already hot and shutting the boiler off soon after the boiler comes on. But, since the room air isn't actually warm, the small bi-metallic spring cools down quickly, causing the thermostat to call for heat again right away. Thus, too much heating by the anticipator will cause the high boiler cycling rate you seem to be noticing.
I think digital thermostats do the same thing electronically, but I don't know anything about digital thermostats, so I can't say for sure.
Maybe investigate the possibility that the heat anticipator setting may be the culprit as well.
http://yarchive.net/electr/thermostats.html