How do I get the filter cap off?!?

I can't remove the filter cap on an Ignis AGW386 w/m. It turns (anticlockwise), clicks and doesn't move any further. Water has drained out on opening, obviously. How do I get the filter out/ cap off to clean/inspect etc?

(I inherited this w/m with no user instructions and can't find a manual to download)

Drum is smelling a bit musty so I'm sure there is something to clean!