How often do plane stowaways fall from the sky?

The Beeb wakes up after under-reporting past cases with this article: BBC News - How often do plane stowaways fall from the sky? Interestingly enough, they don't answer the question.  I've done the maths though: it's one every four years. Just on Richmond borough.

List of reported stowaways reported on Heathrow-bound planes
Year	Provenance	Airline	Number	Condition	Impact/Retrival	Source
2010	Austria	Dubai royal family private jet	1	Alive	LHR	link
1996	India		2	Dead	Sainsbury's building site?	link
2012	South Africa	BA	1	Dead	LHR	link
2001	Bahrein	BA	1	Dead	Homebase car park, Richmond	link
2012			1	Dead	Portman avenue, East Sheen	link
2002	Ghana	Ghana Airways	2	Dead	LHR	link
2002	Ghana	BA		Dead		link
2000			1	Dead	Broadstone farm, Rudwick, Sussex?	link
2002	Uganda	DAS Air Cargo	1	Dead	LHR	link
2007					LHR

See my previous post on the subject:

• Stowaway death is a chilling reminder that the Heathrow runways point directly over London and 2 millions Londoners