By Laura McGuire The rate at which rent prices are rising has slowed, but prices still remain at their most expensive when compared to earnings in over a decade, according to a new report by Zoopla. In London, where shrinking supply has left tenantsfacing historic hikes in monthly costs, rental inflation is up 5.2 per cent against a rise of 15.3 per cent a year ago. Zoopla said the balance between supply and demand has narrowed the most in the capital, with demand 30 per cent lower than a year ago and available supply increasing by the same amount. Today, the average tenancy in London costs £2…