Doesn't the 1.2 gearbox have the tripoid spider/cup arrangement?
The cup end, ought to have a shaft about 4" (10cm) long on the back.. which has a broad groove in it for a snap ring, which is a wire clip that looks like a thick paper clip.
To fit the driveshaft into the the gearbox, you line up the splines on the shaft with the gearbox and then you push the beast home until the snap ring engages with the locking groove.
Then you need to give it a sharp blow to clip the snap-ring in place (basically you're forcing it into the groove by compressing into it, and then expanding again when it's in the groove (and has the space to expand).
My money is they pushed the driveshaft into the gearbox/receiver but then did not snap it into place, so it's fallen out, rather than the groove is so worn it can't hold it in there.
In use, the drivehaft cup is held in place by the snap pin and by the fact that the other end of the driveshaft is bolted at 200Nm to the wheel hub, so the only outward movement possible is space between the inside of the cup and the tripoid spider (aka "knuckle") end of the driveshaft itself (the bit inside the inner CV boot).
It's a gnats amount of clearance (usually) and generally not enough to allow the cup to move away from the gearbox .. I'm sure someone on the Stilo page (similar setup) reckons his driveshafts have no snap-rings on the end... so the cup end of the shaft does not routinely fall out, unless it wasn't fitted 100% in the first place, OR there's enough play in the spider/tripoid/knuckle that it allows the shaft to move out (and the cup to follow it).
If *properly* refitting the cup into the gearbox housing doesn't solve it (the back of the cup has to be fag-paper close to the gearbox housing.. if you're describing the gap as "a millimeter" then it's not in place properly) then have a look at the spider.
You can get a new spider from £20-ish but if the cup is also worn (it gets grooves worn into it from the spider) then a driveshaft is about £70 from J&R .. your labour will be the same, so if you can afford it, a whole shaft is a better idea... you won't have any more aggro' from it for years.
Ralf S.