Shower only hot when bath is run!!

[redface]

Hope someone can help. I have an Ariston combi boiler in a small flat. After a number of problems I had a Canco flush. This resolved some issues but problems still went on with erratic shower temperature. So I then had the pressure balance cartridge changed in the shower (Grohe) and the temperature was constant briefly but now its only constant when the bath is run at the same time. This is driving me mad and Im very concerned that the plumber is clueless.

What would the pro's suggest as the next steps/possible issues?

Thanks

 

[cider]

sounds like the diaphragm washer needs replacing (if it has one)?

 

[redface]

Is that a shower piece? Easily changed or a plumber job?