Electric shower

This recent spurt of steamy weather got me thinking about taking a nice cold shower, but it seems our electric shower has other plans. With the dial at its normal position at 4-5 the water is nice and hot, just right for a normal shower. With the dial at 0 though, the water is nice and hot, just right for a normal shower! I haven't tried it with the dial at 10, for the sake of my skin. Water pressure is fine and I've had the front panel off and a cog on the back of the dial engages another cog on what I assume is a large valve and the cogs mesh together as they should. Before I splash out on a new shower, does anyone know what might be wrong, and does it have a DIY solution? The shower claims to be a Gainsborough Swift.
Matt.
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This came up a few days ago so if you look through previous posts you will find it. I think that was a Gainsborough. Cold showers always seem like a good idea until 30 seconds in you realise cold is too cold!
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I found that thread posted 10/7/03 on google groups. It doesn't seem to be on my ISP news server though.
The suggested problem of the microswitches being seized seems unlikely as the 'Low power' setting is currently selected and hasn't been set to anything else for a couple of years.
Would I be correct to say that electric showers work by sending cold water through a constantly 'on' heater who's power is crudely controlled by the 'high/low power' buttons, and the temperature of the water is increased by reducing the pressure with an adjustable valve thus causing it to flow through the heater more slowly allowing it to heat up for longer? If so, would it even be possible to get a cold or cool shower with no way of turning the heater off?
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