P38 4.6 v8 spat a plug out!

So after my problem with the alarm constantly going off, today i went to start the car up, left it running for 5 mins while i went in the house, and everything was fine. When going to set off, put it into D, pressed accelerator lightly and heard a pop noise followed by clicking.
Turned ignition straight off and popped the bonnet, to find the furthest forward on the right, lead hanging down onto the exhaust manifold, with plug still attached! Looked at the thread with inspection lamp and it looks like theres nothing there really, but it doesnt look like bright metal like its only just happened, so may of been leaking lightly before.
Its got full LR service history so they must of cross threaded it at some point. Just wondering what i can do with it, had a look at helicoils, but does anybody know what size i would need? Also if the threads hadnt already been stripped, and this has happened all of a sudden does anybody know what could of caused this? Has anybody had the same problem, Thanks, Sam

Hi Sam

Dont waste your time with helicoils, find someone that can fit a Wurth Time Sert, they are superb and should be able to fit in place. They have got me out the sh1t loads of times over the years and when installed properly will never come out.

Steve