I purchased, gutted and renovated a 1 bed flat in the city 4 years ago. It's gained about $7,000 a month every month for the last 3 years, plus saving about $3,000 in rent. Not exactly 'income' but most certainly passive, and I do plan to sell out when I feel prices are aproaching a peak and move out the city so will yeild then. Neighbouring flats usually sell within 7 days of going to market.
Haven't seen flats sell as quickly as they are now. Living a 3 minute walk from Liverpool Street station and the flat I wa renting sold by the second viewing. It was on the market for three days but things are cooling off.