I’m not a plumber, but I’ll give it a go as no one else has replied. What you have there is basically a big fancy tap, that will have seals and seats in it the same as any normal tap or valve in the house.
After 8 years it’s probable that one of those has become damaged, (by some scale forming in the seal perhaps?) and the only way you’ll be able to tell is to take it apart and see. If you are lucky it will be a standard size washer or O ring that needs replacing, if not then a specialist plumbers merchants, (Plumb Centre have lots of branches) or Aqualisa supplier should be able to help with bits for an 8 year old shower.
The only thing you need to decide now is do you have the skill to tackle it yourself or get a bloke in? Don’t forget to isolate it if you have a go yourself!