how can i fix my triton marbella shower ?

Hello i hope you can help please i have a triton marbella shower and in the last two weeks the water is very hot in fact scalding and i can't turn it down when i turn the big white temperature knob with the numbers on it to a lower temperature setting the water doesn't go any cooler it stays scalding and i have lots of water pressure that isn't the problem, please can you help, thank you Graham