It'll be inside the gearbox and it's called a concentric slave cylinder, it's part of the clutch kit but requires removal of the gearbox.
I'd bleed the clutch first, then possibly replace the master cylinder and if it is still a problem then you'll be looking at the box out.
You may be able to test the master if you block the pipe that goes to the slave, not done it on these so might need to confirm that.
56 350 2.2 130ps mk7 with 2 side doors
65 290 2.2 125ps custom with 2 side doors
07 350 170ps
Ids v86 licence + v86.04 update + xprog
hereIds map
here