"And Londoners moving away in numbers - when did that happen"

Since London was founded, actually. Families move into London and then, a generation or two later, move out. In recent decades the government has encouraged people to move out to various garden cities and new towns.

I was quite surprised to find that my family could trace it's occupancy of one London street back to 1890, ending in 2006. But we've been moving further and further out since the 1970's.