How high do your bills have to be before insulation is worth the cost?

My heating bills are so low anyway, because I usually only heat one room at a time.

I don't know if it is worth spending hundreds on insulation, when even if it halves my bills, they will still only be around £150/year, and I estimate the costs will mean it takes years to get a return on most insulation investments.

Where is your house leaking heat from mostly?

£500 is not as bad as some, but still probably could be brought down by several £100 per year if you had optimal insulation and boiler efficiency.