your plant is Colletia armata - its a member of the Rhamnaceae which I believe is native to Chile. It is not commonly cultivated but several nurseries (particularly those based down in the milder SW) sell it. Its relative C. cruciata is equally impressively armed and both make for talking points - and hedges that you wouldn't mess with!
Thanks for the speedy response, I have asked everyone I know to no avail and searched the web endlessly. It was only trying to identify some tricky British orchids that my wife Jo came across this excellent resource.