Now they're officially in bed together (more here), Knight Frank Residential and US brokerage Douglas Elliman have decided to pool their resources on the research front too.

Prime rents around the world are rising, on average, by 1.3% a year according to Knight Frank's latest spot check. That's significantly down from the rate of 3.5% recorded a year ago.

247,000 new rental households appeared in London last year alone, says CBRE, as "the emerging build-to-rent market is beginning to bear fruit."