Iain
We haven't met except through these forums but you strike me as a sensible, decent sort of bloke with a bit of experience of the world and I wouldn't presume to give you advice.
Instead let me tell you about how I have dealt with the sort of situation you face. First of all, I've been tempted, as you no doubt are right now, to indulge in the sort of antics that have been mentioned: anonymous nuisance mail, fiddling with the bastard's car etc etc. However, I've resisted. A very wise bloke I once knew told me many years ago that this is a small world, made smaller by the fact that we tend to move in restricted circles. This means, he said, that your actions - for good or ill - will normally have consequences within the group of people you know.
Bearing this in mind, on a few occasions when I've been on the receiving end of the kind of low-life behaviour currently affecting you, I have resisted the temptation to take petty revenge and instead bided my time. Sure enough, each time I've been presented with an opportunity to do the bastard some damage. And each time, I've made the most of the opportunity.
I won't go into details. Suffice it to say that I've never done anything illegal. Mostly it's been a matter of someone approaching me and asking for an opinion about the low-life. I haven't held back - I've given an honest and accurate character reference.
On two occasions I know that this has caused them serious financial inconvenience.
On one occasion, the @rsehole in question found out that it was me who was the cause of his problem. He fronted me - all outraged. I was happy to explain to him that that's the way the world works - what goes around eventually comes around. That particular @rsehole caused major problems for some other people I know. A friend of mine came close to dying from medical problems brought on by stress. I'm happy to tell you that Friend @rsehole's commercial and personal reputation now smells like an old fish and people avoid the bastard like the plague. Bad cess to him!
Col