Poor hot water temperature with heating on?

I have a Vaillant Turbomax combination heating boiler.

When the central heating is operating & a hot tap is opened
the hot water temp is low. If the heating is off then the temp
is fine.
I have noticed that the Diverter valve is still allowing hot water to
flow around the central heating system.
Is there any way to test the Diverter valve ?
Has anyone else had the same problem?
Many Thanks