I think I just ruined my engine

If the engine is spinning freely at speed (noticeably more so than when you normally try to start it), sounds like the cam belt has snapped. The action of the car rolling down the steep slope may in theory have put more torque/stress through the cambelt than normal driving, but it does strike me as unusual.

Is it an 850? You may have got away without damaging the valves/pistons but a new belt aint cheap unless you can do it yourself. Lets hope I am wrong.

Belt was new around 2,000 miles ago- I don't think it would have snapped.

Basing this on the belt being new and 300 horsepower not having snapped it under heavy load.

I can take the cam cover off and have a look and answer that one properly.

However I think if it did anything then it will have slipped, then when I tried to start it things went wrong fast.

These engines will run one tooth off, two teeth off and you find out why they are called interference engines I suspect.