@
cathbloom
Firstly, you will need a good electrician to survey both properties and make sure that once you combine the two, the incoming supply has enough capacity for both.
An electrician will then need to rerun cabling from the second consumer unit back to the first.
Once that had been done, you would need to contact the supplier to have the second meter removed and the MPAN deleted from the database. All the time the second meter is in place, you will be paying standing charges for the supply even if you are not using it.
The supplier will need to contact the DNO to arrange decommissioning, removal or isolating the second supply. It may be that the single supply may require an uprated fuse if it is now supplying what is essentially going to be a single property. But the DNO's engineers should be able to do that without too much difficulty.
As to costs...I have no idea, but probably around two to three years worth of standing charges on the second meter.
I'm rather more familiar with installing a second supply into one property than splitting one supply between two!
It's a bit of a work up but not impossible.