Thanks Robert for your useful input,
further careful examination showed that the utility meter is not optical. So bang goes that idea!
I will have to examine the cables carefully to make sure that they are both single core. The incoming main is a bunched cable, and I remember the electrician suggesting that he would install the same type back to the house. I suppose there might be some writing on the cable if I am lucky. Would you class single phase, bunched as single core?
I think the one from the inverter is single core.
Re. the plug socket:
The main coming in to the farm comes from 2 phases which have been bunched, so they act as one I believe. This supply is then fed to two independent utility meters. My guess is that they are in phase with one another, as they were bunched on arrival at the premises. The difference in voltage between the two supplies will relate to the output from the 5kW wind turbine, and use of cooker, kettle, heat pump in the house. I tested the fall in house voltage from using the cooker and kettle just now, and found that the voltage fell from 239V to 231V. Do you think this matters? This voltage drop would presumably feed back to the other utility meter, what with them both being on the same phase.
Many thanks again for your response.
Mrs. Helga Irvine