How can i un-jam a temperature control on a triton Hp combi shower?

The combi boiler is still working fine, and hot water is coming from my taps, but the shower has all of a sudden (is c.3 years old) got stuck on a tepid temperature. If i turn the temp control left, it will get colder, but if i turn it past the middle it has no effect. I think that the temp control dial is either stuck or slipping, how can I access and free it?

I can get the shower back up to temperature by running a cold tap in the sink to reduce the cold water pressure