Multi stage abseil off top of route (route was approximately 300m high) - fixed belay points. Some navigation error led to much longer descent but still on bolted belays. Weather was very warm and we hadn't brought enough water for trip and generally very dehydrated do decision making felt poor. We also just wore our rockshoes having left our approach shoes at base of the route. Half down the route involved abseiling over an overhanging rock which bulged to the left. As I came down over this the rope swung quickly over to the right into a wall - I put my feet out to stop my body hitting wall which resulted in my foot twisting. At that point it swelled so much we suspected fracture. I managed to descend slowly on knees using prussick to control the speed to more level ground in a gully. We were still 100m up and I was unable to descend so had to ring for mountain rescue. Got airlifted off and taken to hospital. No fracture but level 3 sprain - end of holiday and out of action for 6 weeks.