diverter valve stuck
diverter diaphragm split or stretched
faulty microswitch
Without knowing what you're doing, you'll struggle to diagnose the fault yourself. Call a heating engineer to take a look.
My guess would be the diaphragm in the pressure differential valve has split. If you know what / where on the boiler this is, you may be able to do some more checks. Let me know.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!