Some times grease builds up around the cover which makes it harder to unscrew.
Have you tried use a tee towel to loosen it?
Or spray a bit of WD40 around it and leave it 5 mins to soak in and see if this makes it easier to turn.
If not then you need to get a pair of pipe grips or mogrips on it but not to tight as to break it and that will do the trick.
Once the seal breaks it will unscrew a lot easier from there.
I know it may be a bit tricky getting the pipe grips on as the cover is in the back right corner in the roof but you should be able to do it from there.
Let me know how it goes from there.
I've tried the tea towel & WD40. I'll borrow some grips tomorrow & see if a can do it that way.
Thanks so far.