Hi, thanks for the reply. It is all a little confusing :)
What worries me is that, as mentioned, we took possession of the flat at the start of May (actually 1st). In the 30 days until we got our new Cust Ref No on 30-May, we were topping up as 'guests', which would have appeared to be under the old Cust Ref No of the previous tenant.
So my suspicion is that much of this £45.95 credit was built up by ourselves, in the 1-30 May period. But because our 'move' was not processed until 30-May, it has now been allocated to the previous tenant, and taken from us through the 'debt repayment' system.
Our 'move' should have been backdated to 1-May and the balance on that day should have been allocated to the previous tenant.
What we really need to know is the exact balance on the meter as of midnight 1-May - and for adjustments to be made, if necessary, on the basis of that figure.
((please, please don't tell me I need to 'phone the call centre for this - I have the feeling I'd be on hold for an hour and end up speaking to someone who wouldn't really understand...))