Morning - thanks for all the replies, very helpful. Spoke to local LR dealer and they've sorted me out with a second hand spare wheel (steel rather than alloy) for £20, which they say will deter most. New tyre on its way too. Will make sure the nuts are extra "feckin tight" and will be checking...

Hello all - just to let you know, for those in Ripponden, Sowerby Bridge and around - had the spare wheel and tyre taken from the back of my Defender last night (Sunday 12th December, around 21.30). Has a brand new General Grabber 235/85 R16 tyre (not very expensive) on a 55 LRXS alloy wheel...