Bailey Olympus 2011 Truma Water Heater

I would appreciate some advice concerning the valve which changes the water supply from external to internal on the Truma Water Heater please. Currently the position is set to pull water from the external supply (see photo) and, in order to change this to the internal supply, it needs to be moved 180 deg in an anti-clockwise direction. However, the blue handle refuses to move and I am wondering if I am doing something wrong. I am loath to use too much brute force and ignorance as I don't wish to damage either the handle or the adjacent plastic pipes. If anyone can suggest where I'm going wrong, I'd be most grateful. Thanks in advance.