Was that an instrumental though, in the true sense I mean. If you don't include that, I suppose the next previous one would be the birdy song by the tweets in 1981 (though, again, could that be classed as truly instrumental?), and I think the one before that which WAS truly instrumental was the Simon Park Orchestra with "Eye Level" in 1973.
I think Postdog's right in that the Crazy Frog monstrosity doesn't really count as an instrumental. It may not have any real words, but it does have a vocal of a sort.
I guess the last true instrumental to hit number 1 was Flat Beat by Mr Oizo in 1999. It did have a brief sample of speech at the very start, but there was no vocal accompanying the "melody" (for want of a better word).