Ok so either someone is tapping off your home supply (which fyi shouldn't be three phase, there's absolutely no need for it in a domestic property) or your meter is naffed.
First step is to turn off all power-drawing appliances in the house that are wired to the offending meter and see if it still ticks over - do this when all adjoining property is occupied and in use. If it is someone is nicking the leccy and you're paying for it. If not, the meter could be buggered.
Either way you need a qualified electrician in (check they have Testing and Inspection qualification as they're much more likely to be able to find their way around the system) and after their diagnosis you need to alert your supplier. Either way, you're clearly not using £2k a month in electric with domestic norms like toaster, lights, kettle. You couldn't use that much if you left that all on constantly 100% of the time with that set up.
As an example, you know how if you leave the lid off a kettle it will boil contstantly and never finish? Well if you did that and left a 2600w kettle boiling for 30 straight days your bill would still be less than £300.
I would hope it's a damaged meter because then you have a chance of getting your money back. If it's a horticultural expert you've got little to no chance, they'll pay it off a quid a month.