Programmable TRVs - worth it?

[Johnking]

I'm thinking about fitting some programmable TRVs to the parts of the house that are not used in the day - bedrooms, bathroom (which is especially cold and must be a big heat sink). I found some basic ones for £16 that would do the job. What I can't work out is how much I'm likely to save. there must be a basic cost of running the boiler, then presumably a per-radiator-surface-area cost. My plan would reduce the radiator activity by about 40% but I don't know what the base-cost of boiler running is.

Any idea how I work out what I should save? the overall idea is actually to have the heating on a bit more in the day as we are in a lot this winter.

Cheers,

John