Rentals are still rising at double figures rates on an annual basis and are now 10.5% higher than a year ago.
The data shows that across the country the average tenancy signed during the three month period was £992 per month but in Greater London it was £1,558 per month.
The index report also points out that the trend is particularly marked in areas where rents have been rising exceptionally quickly this year, notably the South East of England and East Anglia.
The biggest fall was in the North East, where rents paid for new tenancies in the three months to August 2015 were, on average, 2.1% lower than in the three months to July.
'On an annualised basis, however, rents in most regions are still significantly higher than the same period a year ago, with only the North West reporting lower rents for new tenancies in the three months to August 2015 than for the same period last year,' he added.
He explained that there is a robust rental market which is consistent right across the UK with only one or two exceptions, such as East Anglia, where prices rose sharply in 2014 and early 2015 but have now slowed notably, and the South West, which continues to see annual price rises in double figures.