Bad luck Andy.
I had the same problem within a week after fitting my Nismo's,right near the sidewall.I ended up buying a new tyre before having the screw removed,luckily when the took it out it had not gone through.So i ended up with a spare front.
The bad news was it turned out some jealous w####r at work was putting them in peoples tyres,several cars got done but never found out who it was.