Shower starts hot, goes cold / luke warm straight away

[SAPFO]

Have and old electric shower. A Gainsborough Style 400x. As title says, I start it, it is hot for a split second, and then goes luke warm. Turning to the cold will make it go then usual cold.

Tap hot water & CH fine

Any ideas? Can I DIY it?

Thanks

 

[flameport]

Given it's age, get rid of it and fit a new shower.
http://www.gainsboroughshowers.co.uk/shower-store/electric-shower/Gainsborough-85-se/

Theoretically repairable but not worthwhile even if the parts could be obtained.

 

[SAPFO]

Cheers. Do you know if this one has the same fittings?

 

[SAPFO]