I guess what we need to know, to fully understand the question is, for the Betfair exchange only, what percentage of clients Back and what percentage Lay? There'll obviously be those that do both (traders), arbers mostly only Lay.

It still comes down to, of the billions that Betfair turnover, half is Backers $ and half Layers $, but as a percentage of clients, I feel that the Layers are a much smaller head count than Backers. A lot of people just don't like or understand the notion of Laying, so the layers do the heavy lifting by turning over much more money than Backers?