how can i fix my T80si?

When i turn on my Triton T80si shower, it srarts to get warm, then goes cold and stays that way. The lower of the 2 thermal cut outs, has been very hot, but my local electrical retailer tells me it is fine. The wire to it had been so hot, it had burned off. I have repaired it, but still have cold water after a short period of running. Can anyone help?