Found this today hiding at the bottom of their site:
Earn a discount voucher for every friend you recommend to Play.com
As a loyal Play.com customer we'd like to give you a reward for every friend you introduce to Play.com
Recommend us to friends, relatives or work colleagues and we will send you and the people you recommend Play.com discount vouchers.
You can refer as many people as you like. For every person who creates an account, they will receive a discount voucher. Once we have posted their first purchase, your discount voucher will be on its way to you!
How does it work?
All you have to do is click on the "Recommend a friend" button and log on to your account.
Enter the names and email addresses of the people you wish to invite and we'll do the rest.
Your friends are under no obligation. We will provide them with details of how to register with Play.com and they can take advantage of the discount in their own time.