I have this problem too with our Redring electric shower. It started happening after I washed the entire shower cubicle using the shower to rinse the suds, and I put the temperature up to the maximum, which caused the 'overheat' light to come on. Also there's a chance that when I was rinsing close to the shower that some water may have seeped inside (although I would've thought that an electric shower would be able to cope with this).
Tom
August 2013