@Slaphead Terrible tale, it could not have gone more wrong, so good luck with sorting it out. You might find Amey more helpful than SP, although the problem lies with SP, unfortunately this sort of thing is reported here quite frequently.
@Accessdenied Surely it can't have been long since you moved in? I mean, it's not as if you've not been looking at your bills or readings for years, is it?
I mean, you MUST have taken a look at your bill and seen the 'E' on it signifying an estimated reading sometime in the last month? Even if you're on quarterly billing, you'll have checked your bill and/or usage at least ONCE in the last three months and seen it was estimated? Didn't you?
If not, why on Earth have you been letting SP send you estimated bills for more than 3 months?