My turn to encounter this problem.
Did the Williams track day at Castle Combe and got the fuel light part way through the afternoon. Drove to local petrol station. Spent the next 10 mins failing to open the filler cap so gave up and headed home early

luckily with enough fuel to make it home.
Spent another 15 mins fiddling with it this morning in the garage and finally managed to get it open. I could get the top to turn but the lower plastic part was not aligning properly so keeps catching. Have greased it but still catching. Will try tightening the nut slightly to see if that keeps alignment better.
Ps. I noticed the image above shows the cap without the retaining cable. Have others removed the cable? It seems to be more of a hindrance to the locking mechanism than a help in keeping the cap attached when open for fuelling.