Need help opening up a Grohe mixer tap to change the main swivel O ring

[Knobbiewilliams]

Hi all, looking for a bit of advice. I have a lovely Grohe kitchen mixer. Sadly, after many years of use, the main O ring seal on the swivel has perished and leaking badly. I've descaled her as much as possible but I am unable to release I guess, the threaded, upper part to expose the O ring. Do I just heat it up or is there a tool available to help get a grip on the parts without scuffing them. Thanks in advance, Rob

 

[ShaunCorbs]

Strap wrench and some heat eg heat gun