How to remove the shower valve cartridge?

Hello, I am an amateur plumber who decided to do a favour for a friend. They have what looks like a Bristan external shower valve but must be old as can't seem to find the model it is. The problem they had was that no hot water was getting through to the shower head. Doing my research, I tried a number of options. I tried adjusting the temp on the valve but turned it up full and still nothing. I then checked the hot filter/valve was clear and it appeared it was. The only other thing I can think of is that it is the cartridge. I wanted to remove it and check it out but once I remove the temp control handle, I am stumped as to how to remove anything else! Any help would be appreciated. Not sure how to post a pic on here. If I work this out, I will do that too!