Does anyone know anything about thermostats and boilers?

[Karbea]

I had my thermostat changed on Friday from an old style one (which I suspected was broken) with a nice shiny Nest.
The engineer fitted it and all was well.
During the evening the house got hotter and hotter even tho I'd set the Nest for a much lower heat. Eventually I couldn't take the heat any longer, and switched the heating off at the boiler, which I know I'm not supposed to do.

I now suspect it wasn't the thermostat that was broken, but something to do with the boiler.
Does anyone have an idea what it might be, and how costly it'll be to,fix?

The boiler is a megaflow.

A megaflo is a kind of hot-water cylinder, it is not a boiler.

More likely there was a setting on the Nest that was wrong, or it was in a cold part of the house, or a room which has a thermostatic radiator valve that is set lower than the thermostat.

Your first approach should be to the person who fitted the Nest.

p.s.

turn the Hot Water timer on your programmer to "off" and see if the radiators cool down.

It should not affect the radiators, but might do if you have a fault on the 3-port valve or the wiring.

I'm currently in bed with flu, but as soon as I'm better I'll give him a call.
I suspect this was a problem before the Nest as our house was always too hot or too cold and I controlled the heat by switching the heat off or on, on the pad programmer thing.

The thermostat is/was in the hall and that room was getting very hot (accurate to what the nest said and my independent thermometer) so it should have turned off/down, shouldn't it?

Will try the water test in a sec.