I just got Scott's Catapult and I am impressed. This is a well engineered kit. Two chimpanzees and a trainee could build it! Sorry, old Trek reference. Anyway, the kit is available through Scott at his Messy Desk Blog. I am not sure what he is charging, but if you can lay your hands on one (or two, or three) of these you should. I wish I could get my missile holes as nice as his.
Now, to try and get my hands on some of Simon's stuff.