Thanks for the comments and replies guys.
In answer to your questions, yes they are an unusual site in this country, and were never officially imported by GM but by independant specialist dealers. Newer trucks are now quite a common site these days, the Dodge Ram being the most common, and Ford F150 Lightnings are imported in right hand drive.
The 'In And Out Burger' sticker was actually put on by the previous owner, who was a Californian guy living over here who had married an English woman. I kept it on there because I went to one whilst visiting the US.
Re your comment Skunksmash about not lowering it, I think I agree now and like the way it sits at the moment, I actually quite like being high up - my daily car is pretty low so it makes a nice change.
Put a set of headers and dual exhaust on the other week, here's some pics. You can see the headers, which for some STRANGE reason I painted silver, wish I'd done them black. The crappy 'high temperature paint' I had sitting round in my garage wasn't so high temp after all and has all peeled off. The centre section is stainless, which I had custom fabricated and has a crossover pipe. Have 2 turbo mufflers and the exhaust now exits behind the rear wheels - which I feel much more comfortable about. Wasn't too keep on the hot exhaust being right below the fuel tank cap!
Since taking this pic I've given the engine bay a good clean with 'Gunk'.