Had this happen to daughters car 2 weeks ago 5 times in space of 2 miles on a '54 plate finesse.
The earlier '03 plate fiesta she used to have and I now use has a split pin through the pin that the slave rod attaches to. Seems the later one has no visible means of retaining the pin and any sideways movement on the clutch pedal makes it pop off.
Get in there with a 3 mm drill bit and drill a hole through the pin and stick a split pin or even a hair-pin through and jobs a good-un !
If you don't fancy drilling it then file 2 ridges either side of the pin and find a suitable circlip to stick on it (or a clip-cap, I suppose would work)
Seems like a stupid omission to me.
Typical graduate engineer idea I suppose !