There are a couple of ways you can check this.

My Account

The first is through the 'Referred Friends' section in 'My Account':

Here you will see your available balance clearly, as shown below.

Shopping Cart

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.