It is not only DNOs  A friend has just been forced by the mortgage provider for his newly purchased private home to merge two previous Supplies into one as a condition of agreeing to provide the mortgage. 

Any idea what the perceived risk is?

I can think of 50 good reasons for mimimizing the number of supplies.

Regulation 411.3.1.1 (2nd para) being one.

As I wrote earlier, I am only 99% certain that my two supplies have a common earth. They are only 10 m apart in the same street, but I cannot be certain.

The transformer in my street has recently been replaced, and it appears to indicate where the next transformers are in the same mains, so there really can be only one cable in the street.

Isn't the risk (of two earths) electrocution rather than burning the house down?

My 50 reasons are the 50 p/day standing charge, much higher than it was originally.

Chris Pearson said:

Isn't the risk (of two earths) electrocution rather than burning the house down?

Both. And they are both accounted for in BS 7671:

• Electrocution if the two earths are not bonded together to form "the same earthing system" (Regulation 411.3.1.1)
  
  
• Fire if diverted LV or HV fault current travels between the two because any protective conductors (or adjoining metalwork) can't carry sufficient current (Regulation 542.1.3.3).

I see, that makes sense.

I see, that makes sense.