Key locking nut

I've just got around to trying to take the spare wheel off the boot of my Defender.

Only problem is, there's a locking nut (with a key, not the socket type). The only likely looking key with the keys I got with the car just turns around in the lock with no effect.

Is there a trick with these things? If I haven't got the key is there any alternative to a drill/grinder?

Many thanks.

have yer tried WD40? it may just be a bit seized if its not bin unlocked fer ages

It doesn't seem seized up but I will give it a squirt.

Are these things supposed to click/engage as you turn the key or do you have to do something else like turn the outside of lock as you turn the key?