I'm worried about my derailleur snapping. The chain isn't running around the bottom pulley, it's running next to it I'm worried about my derailleur snapping. The chain isn't running around the bottom pulley, it's running next to it, scraping against the bottom pulley and the frame.  When I change gears it makes a horrific noise and the bike has ground to a halt without me touching my brakes. This has happened 3 times. 

I am doing the London to Brighton charity bike ride in 29 hours. I don't have time to get it into a shop and I have a 54 mile ride to do and I am worried about my derailleur snapping.  

What should I do?
I wouldn't recommend riding your bike at all until you have a professional look at it. Otherwise you're taking the risk of physically injuring yourself and the people around you, especially on a big ride.