The letting agent says average rents are now £866 per month.
Highest returns on investment are in Wales (6.6%), yields in London are lower, averaging 4.8%, property prices are higher and drag down returns.
However, Central London rents are the highest in the country averaging £2,414 per month.
Rents in Greater London (£1,113) and the South East (£1,105) also top £1,000 per month.
The cheapest rents are in Scotland, where landlords are earning an average £635 per month.
Commercial director Nick Dunning said: "Despite more competitive buy-to-let mortgages encouraging investors to expand their lettings portfolios and increase the supply of rental accommodation on the market, we have on average four or five tenants for every rental property we are marketing.
"Rental values are being underpinned by the increasing number of people living in the private rented sector and rents are likely to continue rising unless supply and demand becomes more aligned."
Separate research by marketers Experian revealed the number of rental properties advertised increased 11.4% in the second quarter of 2013, compared with same period in 2012 – and is increasing at five times the rate of homes for sale in the same period.
Flats accounted for almost half the advertised rentals (47%), and 40% of those were in the capital.
Outside the capital, the number of terraced homes available to let increased – up 16% in the quarter.
Jonathan Westley, at Experian UK & Ireland, said: “Some lenders and estate agents are reporting improvement in the housing market, with the Council of Mortgage Lenders data showing confidence and activity growing strongly at the bottom of the property ladder. Our latest figures do, however, show that this trend is not being seen across the whole of the UK.
“In some regions, the housing market remains challenging for first time buyers, particularly in London, which is constraining activity levels for lower value properties and keeping a lid on prices in many areas. This is reflected in the surge in rental properties on the market.”
