@Ruttly
There is old gas in the tank and when I open the fuel tap, it smels realy bad. So I think I,m in for a thread.
First thing this morning I took te compressor and blown te key hole. Only WD40 came out. The key does not want to turn, not even a tiny bit.
I,v put new WD40 in there and give it more time.

I ordered a Raptor and hope te cap is off by the time it gets here.
Edit:I put a large screwdriver in the keyhole. And tried to turn the lock. I heard the magic click.
The key now turns all the way!

I put a big pair of pliers on the cap with a cloth around it so as not to damage anything.
And now there is also a bit of movement in the cap itself.

The tank isn't opening yet, but we're getting there.