Quick update
I figured it was probably the cartridge as I had hot water elsewhere. I took it out, took a photo and sent it off to the manufacturer (Hansgrohe)to confirm the name of the part. Them and di vapor spare parts website said the same part number but di vapor were £25 cheaper.
Received it the other day, installed it and now have hot water.
Big thanks to everyone here and especially oldbuffer for your detailed advice.
So crossover then!