It should turn one and a half rotations to remove, its common for these to get stuck.I take it you have drained the water first using the small bleed hose to the left?I would rotate it as much as it will go spray WD40 around it and screw it back in, and keep repeating screwing and unscrewing to free it.I would try and avoid from using any tools to force it open but to hands (finger and thumb) holding tight and making short sharp motions anti clockwise should be sufficient to free it.
I have tried all of that. The limited range of turning does not seem to free it. It never "comes out" even a little.
If this doesnt work the only other viable option (to reduce damaging the filter) is to pull the machine out, stand it on its side and remove the pump by accessing the bottom of the machine.
That sounds rather a technical job for a non engineer!
It is probably no more complicated that building a ikea wardrobe.
However I think you will probably find a paperclip or something is binding the filter so its unlikly to come free without damaging the ahndle
Time to call out an engineer?
There is a very good video which can be found here.https://www.youtube.com/watch?v=N7aFdZ9C548
Have a look at the video and see maybe if your neighbour or friend can do it, its about 15 mins work, so make sure you dont end up paying more than £50 if you do call out an engineer
I think i have watched it, but I will try the link again