my mixer shower running cold water

Most likely the hot and cold feed to the mixer shower is the wrong way round. If that isn't the problem the cartridge inside may be letting cold water by, because cold is under higher pressure it will push the hot water away and take priority.

Hi , Just to clarify when you say runs with cold, is that is goes from hot to cold then back to hot/stays cold, so cycling temp? does it run cold only? and by cold do you mean cold for a shower or literally stone cold? It could be a few issues, yes crossed feeds or cartrdige letting by due to pressure. Some other ideas it oculd are;It could also be the flow rate to the shower. Ive had many on a combi where the taps are fine but shower not, pop in a flow reg and shower runs perfect. Also had it where no one has set max temp on cartridge so turned that up, various ways to do it just depends on manufacturer, and all sorted. If you feel the back left of the shower and the rear right to the shower, if you can, where the pipework goes to the unit you should feel the left hot and the right cold. IF the otherway round then its crossed feeds. You could be lucky and the cartidge can be turned 180d or easy access to pipework, if unlucky its going to be a bit more costly.

You may have the hot and cold pipes the wrong way round, or the filter maybe blocked on the hot side

Take the mixer tap off run hot and cold water directly through pipes. See if you got hot water. Most likely you got lime scale build up in your thermostatic value

sounds like one of three things the hot and cold supplies are around the wrong way or there is an imbalance between the hot and cold supplies ie mains water hot and gravity cold or in sufficient flow rate through boiler.