Ouch, that just sucks! 
Some jealous cocksucker or young drunk arschloch did that.
Really, the only way to teach them something is to catch them in the act and beat the sh!t outta them with the baseball bat! Like Vince Vega suggested in Pulp Fiction. 
The other day I caught a 10year old stealing my dust caps (I bought them from Awesome, the locking caps with GTI inscription). I scared him pretty well, but spared his life in the end, though.
somebody has robbed one of mine

that sucks!

But at least you are in GB, so they do not cost much to replace. These were pretty expensive for me, as I am in Switzerland. (My Oettinger remap cost over 1000 GBP).
I had a guy in AlfaRomeo drive into my driver's door last February, there was a dent like this, only worse. My dealer said it is not so expensive to replace, since they just change one part, kinda bolt-on. The guy paid and it was not very expensive (I think a little bit over 1000GBP).