Many opportunities arise. I have IAS open and BF.
The only thing I can guarantee you is you must be quick because they don't last long.

Be sure to always back it first with the bookie before the chance disappears. And remember, you pay 5% commission on your profit on BF so if you can back it early at a good price on BF, and can lay it off later, you pay much much less commission.
Commission is paid on profits of the race, not the bet. eg: You manage to back every horse in the race and stand to win $10 no matter what horse wins, you only pay 5% of the $10, or 50 cents.
If you had $100 @ 9-1, you would win $900 on that bet, which equates to $45 commission.
But,
Since you only made $10 for the race, the commission stands at 50 cents.

My point is: the gap between prices that you get with the bookie must take into account the commission you pay if the bet is won through BF.