Once you've made a deposit to your account and bet at least £10 either in one bet or in a series of smaller bets.
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You'll receive your free £10 within 24 hours, whether you win or lose your bets! Simple as that.
Quote from Betfair terms & conditions
Specific Terms : Matched Bet promotion for new Betfair customers only.
Enter the promotion code detailed when joining Betfair and we'll credit the free bet amount to your account after you've had at least the value of the free bet amount settled. "Settled" means a bet that has been fully accepted by Betfair in a market that has been settled on a result with all due winnings paid out.
If you are eligible, in order to obtain the free bet incentive you must have matched bets with a total value of equal to or more than the free bet offer. Voided bets will not be counted towards your total value.
Betfair will use reasonable endeavours to credit your account with the free bet within 24 hours of settling the full free bet amount on the market where you have placed enough matched bets to meet the qualifying requirement.