Be aware, if the chavvy buggers who were trying it on decide to sue you, they would be within their 'rights'

If you put spikes, electrical wires or barbed wire on property in a position where it potentially could harm someone (even though they deserve it and shouldnt have been climbing your fence) you should display the appropriate warning sign.

I got this advice from a solicitor mate when I was looking at putting spike strips and electrical fencing along the edge of my fence after having stuff nicked from the garden.