Last year I posted a question about my Bosch worchester greenstar boiler going hot cold when running shower . I changed thermostatic mixer but made no difference.. Once the colder weather came in and heating went on it seemed to have cured it self. But now the weather has warmed up and heating off shower is going hot and cold again so I'm hoping someone has some thoughts on what's going on .
From the symptoms it could be a blocked heat exchanger....or the HE temperature sensor.If you're not on a contract sign up for one and call them out after a few weeks.
Rooster...its a given that a Gas Safe engineer must be used .....just make sure your guy is experienced in Worcester Bosch or you may end up paying for parts you dont need.