Thanks PFMaBiSmAd.
I realise that now, but it certainly wasn't an obvious cause when I was searching for the meaning of the Can't fetch message! The message is not exactly helpful, but PHP doesn't plan to change the system soon - see http://bugs.php.net/bug.php?id=51818
I've only just moved to mysqli, and what's missing from the PHP site is a thorough overview and definitions of the basic concepts, such as 'connection', 'statement', 'result'. The PHP notes are often woolly, misleading or just totally missing. If you know of a 'Mysqli for intelligent dummies' article, let me know!
Ian