Well, my local auto electrician has located the source of my constant voltage on the dipped beam circuit.
It seems that it is the remnant of an original DIP/DIM circuit that the MG Metro would have had.
Where the loom emerges from the right hand wing cavity there were 3 Blue/Red cables connected together. That is one from the fuse panel, one for the other side lamp and one......extra !@
The extra one was carrying the voltage and has now been isolated from the loom and the problem is solved !
Now I guess that I must have done something in recent messing about with the headlamps to re-instate this bit of the circuit because it certainly didnt exist when I first got the car.
I have to admit I would never have thought of that despite the fact that when the car was running and I switched on the SIDELIGHTs I did get dimmed dipped beam as well. I just didnt think that it was a feature of the original loom !!