Vaillant VCW sine18 T3 WFH-hot water supply problem?

My boiler will not provide hot water at the tap unless it is running in heating mode when the tap is opened (position 1 on switch) but when the boiler shuts down via the room stat hot water can't be drawn off as the boiler doesn't fire up until the boiler starts up again via the room stat when the room temp drops, the water heating switch during this mode is in the instantaneous position as recommended in the Vaillant instruction manual. (bottom of water heating switch pressed in)

The only other way I can get hot water is to leave the boiler water heating immersion switch on all the time which makes the boiler fire up when hot water is drawn off or the temperature in the storage vessel drops. (top of water heating switch pressed in) this cycle of the boiler firing up continuously 24/7 is quite costly & annoying.

Has anyone any ideas what could be causing this to happen & how it can be rectified so that I can get hot water when the boiler is in heating mode but it has shut down via the room stat?

JB
August 2007