My electric shower, which is about three years old, works perfectly with one major exception - it is only producing lukewarm water. If it was totally cold, I'd assume it had packed up, but it's strange because it's not totally cold - just lukewarm. Why might it be?
At three years it really shouldnt be the element. Fact is that electric showers are a very poor second to those incorporated into hot water systems. The colder the weather in winter with the temperature of the water intake so low, most will only produce a lukewarm shower
It could be to do with the power rating. I have a Mira Sport 8 (ie 8 kW) shower which is still nice and hot even when the mains water supply is very cold. I used to have a 7kW unit and that struggled a bit when the temperature dropped below freezing.
Mine (Redring 8) has a summer and winter setting - I thought they all had - and find that sometimes it gets temporarily stuck on summer setting (half power) when turning the knob. Perhaps yours is stuck permanently.