If anyone is interested, a translation of the original novel is now available to buy in the UK (imported from US). The author is Koji Suzuki, and the publisher is Vertical, ISBN 1 932234 00 4 (which probably makes it the first book the company has published).
A friend who works at Waterstones ordered me a copy. Haven't read it yet, but it's designed very nicely by Chip Kidd, who's best known for his work on DC's coffee table books.
Shame it isn't an omnibus of all three novels in the trilogy, though. The others are called Spiral (I think there's a tv series called Ring: Spiral) and Loop.