Question Author
mxyzptlk - have only part-read the Power of One, but his books are set in Africa or Australia. Wikapaedia can give you more details of all his books (can't wait to get started on the next one, but as I'm in the middle of moving, it will have to wait a little while!).
sallabananas - Tandia is the middle book of a trilogy: Power of One, Tandia and Solomon's Song.
saxy_jag - yes, by chance I found the first book in a charity shop. Courtenay's books seem to be quite hard to get hold of and some are quite expensive to buy new. So the hunt goes on!