Changing the washer in a tap is supposed to be easy. This one has small facets making the spanner slip. Inside the tap is all crusty with limescale buildup. You’re supposed ot be able to unscrew the collar by hand, no chance.
As I write, there is descale fluid soaking. Can you see the bubbles? That’s the limescale dissolving.
I’ll let you know how it goes.
After an hour’s soak it came free with a long spanner. Now it’s re-assembled and runs nicely. The other (cold) tap is showing signs of tightness.