How can I fix my Triton Altair Thermostatic Mixer Shower ?

Mixer shower has been working fine for 2 years, has suddenly developed a problem with the thermostatic control - Now will only run cold or very, very hot!

I believe there's the thermostatic control is installed in the pipework and can be accessed quite easily, am wondering if it could be that it is scaled up. Just don't want to invalidate warranty by opening to access the control if it's unlikely to be easily fixed by de-scaling.

Shower should be under 5 year warranty, but was fitted by a contractor who must have the actual receipt for it and I have been unable to contact - Triton have said they must have the receipt before they will replace/repair it.

Would be very grateful to know if anyone's had similar experience, or knows what the fault is likely to be, and could let me know.

Thanks