Little square surface-mount type from Farnell, IIRC. Soldered wires to the connection pads and epoxied it onto the wax pot. I think I found about 10 ohm did the trick, but if you google this group for my name plus TRV and resistor you may find the original article - late 90s-ish. (Thought I had a copy kicking around but can't lay my hands on it ATM.)
give or take a zero, durrr....
Before I installed central heating, I had a number of gas wall heaters, with proportional control thermostat philes (Drugasar). I taped about 3 1/4W resistors around the phile, and ran them from one of those little wall-warts with switch-selectable output voltage. The switch selection effectively adjusted the setback temperature, by varying the resistor power. I can't recall the values I used now (it was ~10 years ago), but they would in any case be different in different circumstances. You will need to do some experimentation to so how many degrees setback you get from each 1W of power.
The original version used plug-in X10 switches to switch the wall wart on/off, but that was later changed to use programmable alarm outputs driving a relay, when the alarm/home-automation cabling went in.
As Tim says, the logic is inverted, which in my case meant that if there was a power cut, the heaters all came on (not requiring any mains power themselves), but that didn't happen often enough to worry about.
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