I believe from reading SS that if the house has a gas heater and it stops working, you are pretty much obliged to replace it or get it fixed. I don't know if your tenant would accept a different sort of heater that would cost more to run.
Notwithstanding the previous comment, if they are on a month to month lease, would you be allowed to write up a brand new lease starting next month, and write into it that there is no working gas heater? (Maybe have it removed completely?) If they are happy to accept the electric heaters in exchange for no rent increase (because it will cost more to run them) and happy to sign the lease in that format, would that get you out of having to do an expensive repair for such a short time frame before you demolish.
Would it hold up if they take you to tribunal?
Can you ask someone in the field who knows for sure?