Any idea how old this shower is? Does the control in the shower flash red or blue? The earlier ones were red. They came with a five year guarantee but if you are not the original owner probably won't be good anyway.
There is a good chance its the actual control knob at fault and it will have to be replaced. Water would often get into these controls and make them work intermittently. Apart from some of the very first models it was rare for a "grey box" to not work,work,not work etc. If the box was at fault it would normally go down and stay down. There no user replacement parts available for said box.
David
October 2010
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