Electric shower running luke warm

[Chainreaction]

I have an old Berrymagicoal Action 850 inherited with the house over ten years ago. It has always been tempermental but I managed to set it so it was OK and told everyone in the household never to adjust it.

Naturally my teenage son has fiddled with it and it is now just a bearable tepid temperature.

My questions:

1.Is it possible to get a replacement thermostat (if indeed this is the problem) or will the unit need replacing?

2. If replacing I suspect the Berry is 8.5kw ( Action 850?) and given the age the cable will be 6mm so I should stay with 8.5?

3. Electric ...Mira or Triton?...or Thermostatic Mixer? I have a Worcester Junior 24i.

4. I'm on a strict budget so I would be grateful if I could have some rough and ready quotes all in.

Many thanks.