The heads are finally off.
The pacient in the incensive care unit :
The nut that caused trouble is the one behind the manifold
Manifold removed :
About 10 hrs of work total to get them off. Didn't seem like such a tough job.
I removed the injectors with a 28mm ring spanner
Good thing all the nuts I had to remove were made out of stainless and it didn't take much to undo them