Thanks Robbo,

I was on the verge of calling someone out and decided to take one last look and see if there was anything blocking the pipe.

Anyway, I connected a hose to the pipe and took it outside and turned on the mains...

At first, clean cold water came out  - not at the same pressure as the bath or vanity, then something odd happened, the water went hot!

Cut a long story short, there is a very small hot water tank in the kitchen, there is a prv on the cold feed. The cold supply for the shower is fed from after the prv, and not the same run of pipe in the bathroom that all the other pipes are on. So, after destroying the hallway wall, I've now fixed it, and can get back to grouting the floor tiles, something I should have had finished last week