We have tried twisting both ways, pulling etc. No screws visiable. Sorry for awful photo, it's pitch black in the bathroom now!
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The glass part should screw off from the metal part.
If it's stuck, try tapping the glass sharply (as hard as you dare!) with the handle of a large screwdriver, or something similar, while you try to twist it.
It might help to wrap something like a leather belt tightly round the glass, to get a grip on it all the way round.
Be careful if it seems like you are unscrewing the metal part from the ceiling. If that starts to happen, there is a chance you will short circuit the wiring inside and trip the breaker on your house lights when you try to switch this one on.
Try turning it counter-clockwise instead of clockwise like someone suggested before. Also, try to get the glass of first, and then the rest. It seems like you're just having issues with the ceiling :( If it's stuck, maybe try to push it out with something flat and sharp. Maybe a knife would get it moving?