I had no idea these things were in use in the UK; I'd have thought the combination of UK rain and the price of land here would have made them not viable. I came across them on a farm in Australia a few years ago (the above posts just prompted to track down a satellite image of the farm on t'internet - fascinating!
The irrigators I saw were in massive fields at least a mile across - they were essentially a long pipe supported about 20' off the ground by wheeled frameworks every 30 yards or so; each frame had its own motor to drive the wheels, and all synchronised and computer-controlled so that the outermost frame trundled along at quite a lick, whereas the inner ones barely moved. Did 1 rev/day IIRC. And my kids were impressed with the autodialler which would rouse the farmer in the middle of the night to say (in a sexy female voice) "sorry, I'm afraid I've broken down and I need you now..."
David