Thanks guys. The chassis is 15mm thick RG1000 (which is a regenerated grade of UHMWPE, very similar properties but cheaper) from Direct Plastics. In short, it is now my favourite form of plastic as it feels like Nylon but cuts like HDPE
It's all drilled and tapped together, 5mm high-tensile bolts. Only got 8 bolts in there so far just to get it together but there will be a few more added here and there.

That tyre is basically your standard mountain bike tyre but I picked it up from the local skip and it is a bit worn down, but that's quite good because it means the profile is quite flat across the way yet still has a good deal of grip. Would probably be ideal to eliminate the bumpiness in Satanix. I've got over half a bike wheel's worth left and while I want to keep a bit spare, there should be enough for a pair of Satanix' wheels. PM me your address and the diameter of your wheels (or the length of the current pieces) and I'll cut some and pop it in the post for you.