The diverter diaphragm will probably be torn or stretched through use and age. To change the diverter diaphragm firstly turn off the electric supply to the boiler and drain the heating side. Turn off the cold supply and open hot taps and allow to drain.
When you remove the diverter check all ports for limescale and sludge, cleaning as much as possible. The limescale can become quite hard and will be enough to tear the diaphragm. Use silicone grease to grease all moving parts for example the spindles inside the diverter valve.
When you take the valve apart be careful as it will probably be fitted with a spring take apart carefully so you can see where all parts go for re-assembling. Fit the new diverter diaphragm and push the plunger up to make sure it is free to move (hope this is the correct boiler i'm thinking about, will either be a plunger type spindle or a fulcrum arm) anyway pushing the valve up will make the spindle appear from the valve or the fulcrum arm to move down both will operate the microswitch.
I'm pretty sure it will be the spindle that will operate the microswitch on your boiler.
BUT if its a fulcrum arm and it does not move when the plunger is pushed the fulcrum arm will probably need replacing too.
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