The
Luftwaffe Crash Archive series (eight volumes so far, and up to July 1941) is/will be the most comprehensive reference but a rather expensive way to track down a crash when you don't know the date. Single-engine fighters rarely got as far as North Devon, just because of range limitations (although a disorientated Fw 190 pilot did land at Chivenor), so it's more likely you have a piece of a longer-ranging plane. Establishing a year and/or the aircraft type would make your task a lot easier, though.