Wow, sorry to hear your having problems, some people
Are they in the sidewall or tread?
Also, check if the nails look relatively new/clean or are they badly weathered?
Are they always the same size/length?
Make sure you keep them as evidence!
Your either very unlucky, been driving round B&Q during stock take day, or these nails have outside help. Ive been driving around 12 years now, including train yards, work yards, etc. as well as the usual places, and only ever had 1 nail. But that could be I was just lucky though.
I would recomend a cheap cctv
camera with a DVD recorder on long record to watch your car. (cheaper than 4 new tyres) Make sure you put a sign in the window stating you are recording, for legal reasons, doesnt have to be huge, a4 should do. Google images for a cctv
sign to get the idea. You never know, the sign alone might be enough to keep him away.
I believe Home Bargains have a decent enough cctv
camera ATM for around £30
And whenever you move the car from your house, check for nails leaning against the tyres. Easier to let the cars weight do the nailing than trying to hammer them in by yourself.