Do you know of a make of TRV that actually works correctly, all the
ones I've ever come across seem to fail down in one department of the
other?

My ones actually work very well. It helps when they have a limited
temperature mode. I limited mine with a pin to 22C, which means that
uninformed people can't fiddle with them because they're cold and the
heating has only just come on.

My mum's house is all other the shop with temperature. This is largely
because she turns them up when she's cold and down when she's hot, rather
than just putting them on a sensible setting and leaving them alone.

It seems strange to be measuring the 'room' temperature so low down
and so close to the heat source....

Although not ideal, it does actually work. It is helped by the convection
currents that the heating sets up. The TRV (if mounted low down) senses the
temperature of the cold convection air sucked in by the rising hot air.

Christian.