Electric shower

I have a Triton Belzique electric shower (10.5KW). It's about three years old. Problem is - when I press the ON button the heating action does not engage, and thus cold water comes from the shower. After turning the temperature dial a few times the heating mechanism finally engages - and we have hot water. Generally we have to play around with the temperature dial for 2-4 minutes like this - Turn dial to COLD and then switch the shower ON, then switch it off, turn the dial to MEDIUM, then switching ON again (this normally has to be done a few times) - then, sometimes, it produces hot water.

After the shower has been used (ie. it's worked on HOT) it appears to be impossible to get to work again (on HOT) in the next 15 minutes or so.

The water pressure is good, and the water is not particularly 'hard' in our area.

Any ideas.
Diolch / Thanks

I can't decide from what you told me whether its a dodgy pressure switch or the overheat device is turning the power of legitimately or otherwise OR Is it a bit of both?

Whatever ... Electric showers aren't usually worth the faff about fixing them for the price of a new one so if the guarantee is out then just renew it.

Dont waste money getting posh ones with buttons on that you will never use unless you have a huge surplus of cash that needs spending. All the triton 10.5kw showers have the same power element in and thats what heats the water up!
I suppose if they didn't they wouldnt be 10.5Kw showers would they?