1kW does quite a bit of heating to a room...
- typical tough winter is 120 days
- 120 x 1kW x 2hrs = 480units @ 12.5p = £60

Very little over a year (£5/month).
I've assumed 2hrs no thermostat (you may find 20mins is enough).

Failing that, who is for the first wood-burner-storage-heater-combo ?
Heat passes up thro magnetite bricks, ok for 920-980oC?
Ok, probably worse than the firelighter on a timer.

OP - You sure there aren't easier ways to burn the bank down BTW? :-)
Like elect Gordon Brown for another term (or even elect him at all
come to think of it).