Thanks for your responses guys. I finally stumbled across the answer on a Debian site yesterday and I don't know whether to laugh or cry.
Unbelieveably (or not, take your pick) Debian-based Linux versions actually *read* the text in the MBR. If they don't like what they see, they do strange things. Use a disk editor to blank out the words 'Smart Boot Manager' and everything works fine. I have to say this is the strangest one I've encountered yet.
Other than this little idiosyncrasy (which could be argued to belong to Xandros), SBM is a wonderfully intelligent and versatile boot manager that I have used with great pleasure for many years. It is OS independant and fits entirely in the MBR so can be installed direct to the hard drive. It certainly doesn't have to be run from floppy or CD, though it can be. What makes it so good is its configurability. Since it can hide or unhide partitions on the fly as it boots and also automatically change the active partition, multiple Windows versions such as XP and 98SE can peacefully co-exist on the same drive, along with Linux. This isn't possible via Lilo, since that makes all Windows partitions visible to each other.