Ten minutes of crawling under my desk untangling cables and adding the new Belkin cables (they also do sound and additional USB devices) and things were ready to go. Press a button and I'm on a different machine, press another and it's back again. I don't even need to reach down to the KVM - it'll recognise certain key combinations as instructions to switch machines.
(One thing to note - the KVM is unable to cope with multiplexed USB devices, so I can use either the mouse from my Wireless Laser Desktop 6000 or the keyboard - not both. I think I might be able to hack it with two wireless USB dongles, but that's experimentation for another day.)
Expect to see the old four port KVM and some coaxial VGA cables on eBay soon!