mixer Shower doesn't let through cold water

[Lampy]

Hi i have a triton mimosa mixer shower in my bathroom.
It's combined with the bath and pipes built in behind a wall so not easy to replace. When i turn the temp control round to cold nothing comes out at all so i'm thinking that perhaps it's the thermostatic cartridge. Sadly that part is discontinued and hard to find. Found one place that has one left but it's a whopping £110. Could it possibly be anything else worth checking or doing? If it is the thermostastic valve can they be serviced? Cold and hot water run fine in the bath below by the way. Thanks

 

[armyash]

None return valve sticking on the shower?. (if it has none return valves)