jane1966:We believe that this has not been our error and I have read somewhere that they can only claim back for the past 12 months if they have not billed us or used low estimates . Is this true......
.... and is this their error
You are referring to something called the Code of Practice for Accurate Billing. I suggest that you refer to it, in particular the FAQ. The circumstances where it applies are described in clauses (d) - (i). There are also 3 clauses (a) to (c) which apply to you before the supplier is bound by the Code and only you here can decide where you sit on (a) to (c), but if you have ignored a "low estimate" contained in a received bill or statement then its unlikely the Code applies in your favour.
Usually in response to posts in this forum I respond "on the face of it" the Code applies. TBH you will get much better help here by being "complete" with the background. Energy is sold by the "meter reading", whether right or wrong, accurate or estimated. In a long post not a single meter reading, so its impossible to comment on the second question.
That said, the bit about "the meter had been incorrectly read and you owed a further £3500" is interesting. "On the face of it" (in respect of the £3500 only) it's quite possible that you have a case under the Code. Whether that is likely for the remainder of the bill "on the face of it", no.