only cold water in my shower

[londoner]

i have a thermostatic shower mixer valve with a temperature limiter (not sure what type of mixer) that will no longer give me hot water

i checked all other hot water taps - they call hot water at the boiler so it isn't a boiler problem. the shower will not give a hot water demand at the boiler regardless of how long i leave the shower on or what temp i set the mixer to, so i expect there's a problem with the mixer valve but don't know if i need a new valve or whether there might be an easier or cheaper solution. anybody got any suggestions?