Since they fixed it last week, the shower is either scalding hot or freezing cold. It's just a mixer basically, not an electric or power shower.
Plumber says it's not his problem and I'll need to buy a new valve from the manufacturer.
Not helpful as it's a pretty standard shower with no manufacturer details on it! And this wasn't a problem before - it's only been used about 5 times because as soon as it started leaking we had to stop. I've given them £500 and still don't have a useable shower
We have a similar problem, but the plumber mixed up the hot and cold pipes. The thermostat is therefore on the cold pipe so the water is scalding not as it tries to heat up constantly. If you switch it on, it should be the left hand side of the mixer that gets hot. Might be worth checking. Been waiting for a month for the plumber to come and change it, he doesn’t care now he’s got his money...
I don't know how that works but I presume the shrouds come off in some way.
Good news is that it doesn't look tiled in, which obstructs repairs.
Unless you can find out the maker and get the installation/maintenance instructions, you are looking for personal recommendations of an experienced plumber.
Inside the casing is probably a mixer. Hot water inlet one side, cold water inlet the other, water outlet top and bottom, control cartridge goes inside the body. Examples of another brand on here, yours may be quite different.
My guess is that the hot and cold connections have been accidentally reversed. An experienced plumber should recognise the problem and know how to fix it. With luck you won't need any new parts. You could send photos to Lunns and to Shower Doctor to ask if they recognise it and stock parts to suit.