Bosch 1600 Exxcel Express washing machine pump filter cap is stuck?

I cannot turn the cap more than 1/4 clockwise or anti-clockwise. It is stuck. I'm not sure which direction is the right way and don't want to force it. The machine is making a clanking noise when a cycle starts and I am hoping clearing out the pump filter will fix it. I would be grateful just to know which direction to turn the cap and if anyone has any tricks to help as it seems so stiff. Thanks.