Ok! Managed it. I unscrew the pulley beneath the 24 mm nut.
Easy! An Allen key - one screw only.
Then i could fix a 15 mm spanner on the nut behind the pulley with 24 mm nut.
(I used a regular 15 degree angle spanner).
I turned this until the 15 mm spanner stopped against the vedged belt pulley to the right. Used a wood chip to prevent marks (probably not necessary).
Then I linked another spanner eye to the 24 mm c-side spanner to gain leverage. Turned COUNTER CLOLCK WISE!
I thightened the nut quite hard. Then screw the Allen-screw pulley back on.
From above i wedged a long stick between the 24mm eye on the nut, to secure it from slipping, as the grip with my spanner was a bit shallow.
Then - pulled towards the radiator to pry back the spring - with same leverage. Now with the nut-issue fixed - there was no problem to hold down the spring and fit the belt.
Make sure the belt is placed the right way and well aligned on pulleys all the way round. Especially the one up left seen from underneath facing the car.
Don’t forget to remove stick and tools
Pardon my english, i’m norwegian
Thanks! I would not make it without this forum post.
Images showing correct belt pattern and my view angle as written.