Modern vehicles are just so much shite

VimFuego

will give it a go (not that I have any diesel knocking around, lot of petrol, no diesel tho). Can't make it any worse than it is.

ESBlonde

VimFuego said:

will give it a go (not that I have any diesel knocking around, lot of petrol, no diesel tho). Can't make it any worse than it is.

Petrol is not the same sadly although any oil will play a part, there is a rust eating quality to diesel (cant be bothered to look it up) and lots of renovators soak old rusty engineering components sometimes for months in diesel to soften the hold of rust on seized components. You'll have to wear your hoody again and syphon a neighbours tank at night, it's not that you'll need a lot just keep it wetted. 

Edit.

Apparently anything a bit acid can work too, like citrous fruit acid or vinegar (or cola?) so put the dark hood down again and visit the kitchen (take yer boots off first remember).

Also, both before and after you have soaked it in your chosen magic fluid, smack it on the head with a hammer like it was a nail. The shock opens up little cracks to let in the fluid and again breaks the hold ready to undo. And no mole grips or adjustable spanners either, a proper quality socket or spanner so you don't just round it off.