From cold, turn on hot water, feel 22 mm copper pipe (left hand one) if this starts getting hot your diverter valve is sticking. Could be the valve or actuator head.
I turned on the hot tap and filled up the kitchen sink. This pipe ('CH Flow' on Page 6 of http://www.idealboilers.com/docs/pdf/isar_he_manual.pdf) did NOT get hot or even start warming up.
What does this suggest? Is there anything else that I can check?