How does the filter plug undo on my bosch washing machine?

I have a bosch classicxx 1400 express, washing machine. It did not drain after a wash, i have got most of the water out with the aid of the small black drain hose and want to take the filter out to check that its not bunged up, the only problem is, i can't undo the filter plug. It turns clockwise for just under a quarter of a turn and then nothing. It wont turn any further or pull out.