To add to that re £60...
- Tesco firelighter is 11.4p, 120 days is £13.68
- 2kW out of wood is say 10p, 120 days is £12

So that is £26 vs £60 for a 1kW fan heater on 100% for 2hrs.
So the penalty is £34 extra or less than £3/month.

If the price of wood is higher, adjust accordingly.