Image Description: The third wheel fitted onto the leg mount of the Sentinal perfectly (I had thought that I would need the chassis of the SM Attack Bike, but no need!), and I've added the Sentinal's chainsword to give the bike a Boudica-style leg slicer.
Thanks very much Stormcallers, I'm very pleased with how it turned out in the end. I did have some trouble in actually connecting the sidecar with the bike, but I resolved that by using Sentinal leg segments as cross braces - don't want to waste any components after all!