How can I fix my Redring plus 7 shower?

My redring shower is constantly on hot, turning the dials does nothing to change the temperature like it use to. It has high and low on one dial but turning this only makes it very cold or very hot and the other dial is the temperature but when you turn this it just keeps turning and does not change anything it is constantly on the same temp. The shower is a good age but has always been a good one until now and I cannot replace it as my ex tiled round it.