Gravity Shower Boiling Hot No Cold Flow

Hi. Just wondered if anyone can help. I have a gravity fed shower where the twin control thermostatic valve is fitted into the wall behind the tiles. For some time now I have been unable to use the shower as the temperature is ridiculously hot. I have tried reducing the maximum temperature using the override facility and have noticed that when I reduce the flow of hot water coming through, it starts to trickle out showing that there is no cold water coming through at all. Just wondered if anyone can advise if it sounds like it could be something to do with the thermostatic cartridge or something else. I was going to remove the plate, descale the cartridge and see if that works but I'm no plumber and dont know if this would even affect the cold water supply and dont want to remove the plate if unnecessary.

Any help would be much appreciated.
Many Thanks.

 

Well, I tried the above and it looks like it's not the cartridge. There is no cold water getting to it whatsoever. I've removed the tiling to expose where the cold feed comes into the valve and there is a very large screw which I am unsure is blocking the supply or not. It is an extremely large screw which I am unable to turn (not helped by the fact that the idiots who fitted it had filled it over with tile adhesive and tiled over it. I have managed to get rid of most of the tile adhesive but it won't turn with a normal screw driver. Does anyone know if it could be this screw that is causing the problem? If so, is there a special tool/giant screw driver I can get that would be able to turn this screw? Also, If you can see the angle that the screw is turned, does this appear correct or should it be completely horizontal? The cold has obviously worked in the past and I'm sure this hasn't turned on it's own but should it have been left horizontal to stop this happening or is it positoned correctly to only allow a small amount of cold water to flow through?

 

Yes. Tried it. No cold flow at all.

 

Yes removed cartridge, turned mains cold back on and no water at all