How do I clean the filter on my BOSCH WFK5730 ?

I need to clean the filter on my washing machine but it's second hand so I have no user manual.

I have taken the cover off the front below the door and found the pipe that drains it. Yuk that water was awful!

I then uncrewed the large cover on the left (looks like a fuel cap on a car) and more nasty water came out... but I can't actually get the cap to come out.

Is this the filter or is there something somewhere else?