Sounds like the solenoid valve isn't sealing properly.
For all shower issues the website showerdoc.com is absolutely invaluable.
Find the Spares page. Select the make and search for your exact model.
On the image I'd spares for your model there's also a FAQ section that might well answer your question.
The clear diagrams should also help with how to do it.
I think this is he relevant Spares page for your model:
https://www.showerdoc.com/triton-aquatronic-1-shower-spares
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.