triton topaz t100i not getting very hot water?

hello i have a triton topaz t100i shower which has not been getting very hot for the last month or so the dial will only turn up to number 7 and the water is only warm. if i try to turn it up any more it starts clicking and shuts off. i have just checked the elements and the one on the left reads 12.4 ohms and the one on the right looking into the shower from the front reads 16.8 ohms. i have checked the filter where the water pipe is connected at the bottom and the gauze inside has a hole in it so there is no chance of a blockage, can you help please? do i need a new shower or do i need to replace either of the elements and if so is it a big job? many thanks in advance, graham liverpool