Ok.
A (one) definitive answer. It was a piano.
Just been played on BBC Radio 2 (15:45 local time) during an interview with Bernard Cribbins. Bernard confirmed the author wrote it following the delivery of a piano to a basement flat.

The basement bit also answer's the question (from 2005):
What I could never understand was: if they want to "drop the blighter through" (presumably downwards), then why do they remove the ceiling ABOVE it??!