thanks puddle/tom that's helped a lot. i'm not a plumber and was stuck after hot water wouldn't stop flowing & replacement shower (mira) had no hot pressure. still have hydroflo in storage & see some people fixed problem by stripping down, soaking in vinegar to get rid of deposits & lubricating spring mechanism with silicon grease. or i can fit a trevi boost. or a combi boiler! appreciate your help fellas.
Trevis are okay but really need regular maintenance. I cleaned one out got it working then the flow dropped off again 10 minutes cleaned what I thought was okay and it worked fine still does according to the client who I see in a local shop.