I managed to lock up the back wheel earlier in top gear (riding slow fortunately). When I stopped the chain tensioner was jammed in the cassette! I suspect it was because of folding or unfolding it earlier incorrectly. So I managed to fix it by breaking the chain and unscrewing it then putting it back on then joining the chain back up. I've set it so it's locked in position and above the chain in all gears. But it seems very high in 1st and 2nd gears; almost vertical. Should it be able to pivot ie not be tightly screwed up? Should it be almost vertical in lower gears? It's a Shimano 105 derailleur and Hyperglide cassette. It's bent in a curved shape now - all the ones I've seen in photos are completely straight. I'll try to attach a photo.