Funnily enough SEWater rang me this afternoon, regarding a leak allowance. They are coming to do another reading now the loo is repaired, to see what our average usage is.
We've had a similar problem with a faulty overflow which went back into the toilet bowl. Our bill is over £200 so not as drastic as yours.
We do get a bill every 6 months so the August bill showed a marked difference to our February bill.