Thankfully, all I was doing was swapping out my spare wheel as the tread on the nearside front had become illegal. Thats when I found out the spare wheel securing bolt/assembly was totally siezed. It took me over an hour with the aid of a blowlamp, plenty of WD40, a whacking great pair of grips and the assistance of a well built wife to get the thing free. Even then it was severly bent and had to be replaced with another from the scrap yard.
I replaced the bolt with one with a proper hex head as the end of the tool that's used to unscrew the spare wheel broke off in my attempts to remove the bolt.
Tip if yours is siezed too - push up on the bolt from underneath the vehicle with a jack or something similar (you can let some air out of the spare tyre if it is the right way up) which allows you to get enough room in the boot to get a hold of the head of the bolt with a mole grip.