If your metre hasn't worked for a long time TT and they haven't replaced it then I don't know on what grounds they can charge you for electicity you have used, apart from going by an estimate (if they can do that). It may well work in your favour, you may actually pay less than you have actually used this year, or more, but who knows?
My estimates have mostly exceeded what I have actually used, but I have lived in properties where it has been less.
It might be worth getting out last years statements (if you feel you have used the same amount of electricity, this time last year, this year) and asking that they charge you the rate of electricity that you used then? However the cost of electricty per unit may have gone up since then. I would go through your bills and see if what they are charging you now would be less/more than what you have paid last year and make a judgement based on what costs you less for this year, if that makes sense?! It might be worth checkign with the CAB to see what your rights are. You might be in a position to save a few pennies based on their error?