How are you getting on with this problem?
As above first thoughts would point me in the direction of the secondary plate heat exchanger/plate heat exchanger. Keep an eye on the burner LED on the user control panel when your demanding the hot water, if the burner LED stays lit and the water is still cool its possible that one half of the plate is worn out.
If the burner LED comes on, gives you a short burst of hot water and then the LED distinguishes after a minute or so then the plate could be restricted/blocked/has deposits attached to the inside of its plates. This can be a common problem for most indirect combination boilers, as for the diverter unit its a simple check.
When hot water is in demand and the burner LED is lit check carefully on the pipe furthest on your left underneath the boiler (more than likely a 22mm copper pipe), if the temperature continues to rise then it's possible that the diverter cartridge or Unit is passing by.
In this case the hot water temperature will reduce until the rest of the system heats up. Other symptoms from the cartridge being faulty is your heating coming on without you calling for it as it fires up during preheat unless you have the preheat disable kit fitted.
Olly