I've got my own answer, the circlip is on top of the arm, it's very thin and about 1/2 an inch in diameter, so if that part of the car is dirty it might be hard to see it. Removed the thing (don't lose it!), removed the arm without removing the slave cylinder, which is actually quite easy (the fact that I had opened the brake fluid reservoir might have helped).
Then I've removed gently the bush, dripped about 25 drops of HD MTB chain oil down the shaft (5-7 every 90°), generously greased the top bush with HD MTB grease, then reassembled the whole thing (! careful, you have to keep the slave cylinder piston in if you want to reassemble the arm easily).
I've only driven about a mile to get back to my garage, but it already seems MILES better, no notchiness, clutch much lighter and it seems like less vibrations when releasing the clutch. The big test will be on the way to work tomorrow though.
for this manipulation, as said, it just works
And thank you for you patience in helping me