Leeh123:
You should Google for the Code of Practice for Accurate Billing. There are several relevant clauses to your case. Firstly a requirement for estimating based on either previous consumption or average consumption for your type of property. From what you say EDF have totally failed on that clause. Secondly, where there has been supplier error, back billing (legally) cannot be for more than one year. No bill would be a black and white case, this is less clear, but I think they may have failed the spirit of the clause. Other clauses relate to handling of vulnerable customers. From what you say the meter readers (who are probably not EDF employess but meter reading company employees) appear to have been inept at obtaining access to a registered disabled customer. Regardless of employment, EDF are absolutely responsible for managing the customer. In the circumsstances a visit arranged and confirmed by letter or telephone would have been appropriate.
You should ask for proposals for settlement in accordance with the Code of Accurate Billing based on their inaccurate estimating and inept attempts at obtaining access.
Its not impossible that the there is an error in the actual reading which resulted in the huge bill. Try to get a friend to read your meter and ideally take a photograph. Check that the number is close to the bill reading.