I have a completely knackered elbow and need to immobilise it and then protect it from the rigours of mountain biking.

If you're looking to make something that is going to give you really good protection, I would be using mostly kevlar, ie. 2 layers of 240gsm, 3 layers of 400gsm, 2 layers of 240gsm. All layed at +/- 0/45/90 degrees to the previous layer. Diolen is a bit softer, so if looks aren't a factor then I'd probably leave it out, maybe just paint the final product. It's hard to say exactly how many layers you should use without knowing your exact requirements, but really your layup is totally up to you and what you want, you could throw in a layer of carbon in there to stiffen it up, you could put a layer of diolen on each side for looks/abrasion resistance, you could do more layers to make it stronger, you could do less layers if it's not too necessary. You could even do a layer of hybrid carbon/kevlar, which will give you some of the advantages of each. What is important for impact resistance is that you orientate the fibres in as many directions as possible.