Northern Spark said:

The flat was two separate flats that have had a door put in between them, essentially making them one flat.

Can it remain as two separate installations? If not I foresee some serious re-wiring and "gas plumbing" ?

We wouldn't want (if that is a risk) PEN faults (or similar) using the two (formerly separate) gas installations as an electrical path for diverted neutral currents or fault currents that are not expected?

It's not possible to simply bond two electricity supplies together without having a conversation with the DNO. There's a Reg for it (believe it or not) that makes the 'designer' (person who decides to bond them together I guess) responsible: see Regulation 542.1.3.3. At the same time, don't forget Regulation 411.3.1.1 ...

That's going to be a lot of daily standing charges, with two gas supplies and two electricity supplies.  Why doesn't someone do it properly?

Both supplies are PME. Diverted neutral currents is a main concern. Doesn't keeping them separate pose the risk of touch voltages above 50V occurring as some of the extraneous parts will not rise in voltage under fault conditions?

Usual landlord nonsense. 

If the occupier is content with two lots of standing charges, why change anything unless there are simultaneously accessible conductive parts? It does not seem very different from two flats off a common landing.

If the aim is to avoid standing charges, there will be only one of each supply, so no requirement to bond them together.

Perhaps the aim is to allow the flat to be redivided if necessary in future?

If you were going to connect the two supplies, you would want to be certain that they came off the same transformer.

If it's a rental property, then the occupier has no say in the matter.

Northern Spark said:

Doesn't keeping them separate pose the risk of touch voltages above 50V occurring as some of the extraneous parts will not rise in voltage under fault conditions?

Yes, quite simply, that is not permitted by Regulation 411.3.1.1

But because of diverted neutral currents, and Regulation 542.1.3.3, you will need the DNO's permission to bond them, and know how to choose an appropriately sized conductor.

It may well be that the flat needs to be converted to a single supply, or one or both supplies converting to TT.

Perhaps the easiest is to make sure that sockets and radiators on metal pipes, for the two halves are not easily within touching distance (a couple of metres ) of each other - tape measure through the doorway and all that. Any light switch can be the plastic kind where the screw heads are covered.

It may end up being be far easier to move or blank off a socket than it is to combine the installations. It rather depends what is near the linking door. As others have said it is not without precedent with flats that share an entrance hallway and so on. It is possible for someone to bring an extension lead from one side to the other of course, but things on long cords like vacuum cleaners, lawnmowers, jetwashers and so on are normally double insulated these days and if there are enough sockets on each side, why would they need an extension lead at all?

And if they are the sort of tenant who unscrews the cooker point to get a high current supply for something like a welder, well there are probably other issues and this is the least of them.

At some point pragmatism has to be used.

Mike.