Consider this gem from Betfair support (from the allowance thread at the Betfair forum).

"However, the 1000 allowance would have already come into play around 59 weeks ago."

Now if this is true, it means that many customers who were historically not affected by the charge, and have nearly used up their allowance will get charged on the 1st live week. All they need to get charged is an above average week.

So the proportion of affected customers will immediately grow.

I can hardly wait until tomorrow when customers' charges will be revealed, to find the furore from formerly unaffected customers now getting slugged.