Don't know for sure on your Pug, but usually these nuts are tightened by holding the piston strut with an allen key in the centre of the rod (ie the threaded bit stucking through the nut) - there is no other 'counter nut' involved. Or alternatively the piston rod end may have a hex profile you could hold with a smaller socket or spanner whilst tightening the large nut.. Yes it should be tight - don't know exact figure for yours, but as a rough guide on my cit xsara the torque setting is 33 ft/lbs.
These nuts usually have nyloc inserts to stop them undoing (as there are small rotational forces involved on the top mounting of the strut related to normal steering and suspension movement. If the nut is turning freely, my guess would be that perhaps the nut has been reused a few time in the past, compromising the nyloc material, allowing the nut to loosen a little and the normal up & down forces applied to the top mounting (almost akin to a slide hammer) have stripped or damaged the thread of either the nut (if you are lucky) or the piston rod (if unlucky
). Either way you will need to remove the nut pronto to check, and replace the damaged item(s). The nut is what holds the strut/spring assembly together, and it is probably just the weight of the car compressing the spring which is stopping the whole thing from separating! I would certainly avoid any driving which might allow the suspension to fully extend - or indeed jack up that corner of the car which would have the same effect - until it is sorted!Hope the solution is the cheaper/easier of the scenarios above!
Good luck,
Greg