Underneath a selection of crusty doors, engine parts, found two Maltese Cross grills and a small but fairly heavy cardboard box which resisted moving very far, even when encouraged with judcious application of the toe of my boot.
Closer examination, it was still stapled closed, with a small blue sticker marked RTC8020 November 1998.
Inside, a brand new, untouched and complete Overdrive unit.............

As I recall, I only paid around £50 for the whole lot at the time, most of what is only scrap now anyway. First time I have ever been glad it was raining.