The answer to this question is different for referrers and referees (more information about what we mean by this here).
If you are a referrer, you will have to wait until the person you shared your referral code with buys a Refill before the referral credit gets added to your account. When this happens the reward should be added to your "Wallet" immediately. You can check your Wallet in Settings > Promotions.
If you are a referee, the reward should be added to your account immediately. This will allow you apply the £15 discount on a Refill right away!