Hi James.
I can see from your posting history that you seem to like asking the same questions and ignoring the answers – but –
If you’re going to try the DH course at Stile Cop on Cannock Chase just ride down it slowly a few times with the bike you’ve got. If you select big ring at the front and middle ring at the back your chain will be quite taut and probably won’t fall off.
As you get faster and more confident you will start taking off on some of the ramps. That’s fine, but the first thing I would expect to happen is that your fork will start to make nasty clunking noises and develop a bit of play. Probably best to take it easy when that happens.
Try and chat to some of the regulars there and listen to their advice. If you watch listen and learn somebody might even let you have a go on his bike so you can decide whether the extra £2000 it cost is worth it.