There are a couple of ways you can check this.
The first is through the 'Referred Friends' section in 'My Account': https://www.appleyardflowers.com/awraf/index/stats/
Here you will see your available balance clearly, as shown below.
You can see your available balance in the shopping cart once your total amount equals £40 or above.
You will see a new box appear for your referral credit. From here you can enter the amount of referral credit you want to be taken off your total amount.