Intermitent hot water?

have a s/duval 30e system........problem is heating works fine....

hot water....will only ignite and heat.. approx 2hrs after the heating system has been activated..

have cleaned and checked what I think to be the diverter valve...located btm left as looked at (released by means of clip and left hand part of main cylinder pulls away)inside main cylinder.. bar can be pushed and is free...