Water always running out of pipe?

[opps]

Thanks for the last photo. I am not plumber, and am even more confused about the set up, but hopefully a proper plumber will be able to help you.

 

[CBW]

Here’s what it looks like coming out of the wall.
View attachment 279594

As I suspected in post #11 that’s a tee near the tap. I think you may need to trace the grey pipe. Combi should have its own prv.

 

[sjmac]

Cold main going to shed/outhouse with water heater fitted?

 

[Hugh Jaleak]

Looks like a feed to elsewhere, do you have an outbuilding or garage with a water supply? Odd setup though, could stop the water wastage for now by turning off the Ballofix valve a quarter turn with a screwdriver. (That is assuming the supply is coming from the building at that point and not to it!)