i had the same problem. when i tried to change the oil a couple of go's ago i found that the rear nuts turned around in their holes, like the thread was broken. in fact what it is is there is a part that acts as the thread that fits inside the hole and that had broken. The rear ones can be fixed but the front ones will be a ***** if they go. the two side little bolts are also a pig if they spin around aswell. they are fitted to a panel that covers the wheel arch.
Anyway i payed £180 for a new tray when i had had enough of trying to fix it and thought i would let them do it. thinking that they would replace the threaded bolts (which were all badly worn), how wrong was I? What they in fact did was use something to lever one of the rear bolts down while using the air gun to remove it. The other one was a real knightmare! They couldnt get it back in after levering it out so what they did was DRILL A HOLE into my new tray and straight into the chassis!!! I found this out because the next day after i was going to do an oil change but didnt want all the hassel i had to go through previous times. well it didnt matter i went through it again anyway and found out i had a new F***ING hole in my chassis ready for all the new elements to start to make it rust. the bolt they used was in fact a self tapping screw (with no grease or anything to protect it) which was about 3mm in thickness so it would have rusted in no time and i would have been back to square one.
Needless to say i went ape **** at the manger.at first i was cool and collected until he tried to fob me off with something that i cant remember, so i went mad and said that "you dont bloody drill holes in my car without my permission and i want it fixed and all the bolts replaced!", at which point he said "how about you bring the car in on saturday at your conveniance and i'll fix it while you wait". So they did. now its fine.
So the moral is CHECK THE WORK after its been done! make sure you tell them to change the bolts and you dont want them spinning around in the holes when you come to changing the oil next time.