Triton T300 si. Can't now control the heat, only 2 years old?

Shower goes from cold to very hot can't adjust to suit.
Had plumber in who said get a new shower. Am reluctant as it's only 2 years old and Which says Triton is one of the best showers on the market and I'd expect it to last a lot longer than 2 years.

Marjorie Archibald
April 2009