Vaillant vcw gb242 eh central heating problem?

It isn't the diverter, it'll be the servo valve not returning to its rest position, those little tubes will point you to the servo.
The most likely cause of it not returning will be a leaking water valve through the stuffing box, this 'gums up' the push rod that operates the servo when the domestic water is used.

New stuffing box/water valve & clean the push rod (in the bracket that the servo sits on), Bob's your mothers brother

Carefree
June 2019