[philipwhiuk]

I thought it might be an jet-engine cowl, but I don't know enough about planes.

[jamieplex]

Cowlings are generally thin aluminum or composite materials, never steel. Cowlings are considered "fairings", generally to smooth the air-stream in and around the engine.

[buildsjets]

Do you have a theory on where the rest of the airplane went?

[philipwhiuk]

My assumption was that it landed sans-cowl.

But it turns out it might be from an Ariane upper stage after all.