Here’s a tip.
When fitting the new clutch slave cylinder, I found that the spring pressure against the pushrod would keep forcing the cylinder out again, while I was trying to fit the retaining circlip. It’s quite hard to push it into position and hold it with one hand, while working the circlip pliers with the other.
So to hold it in position, I fitted a longer bolt into the front bolt hole for the cover. This is threaded right through the casting, and so the longer bolt can jam against the slave cylinder body. So once it’s in position, tighten the bolt to hold it and that leaves you to concentrate on the circlip.