PayPal interfering with cookie?

I'm using the User Referral module with the Forward module and emailing links to referred members. Before a visitor can become a member of the site they need to purchase an Ubercart product which then assigns then a role with an expiry. I'm using PayPal website payments standard.

When I was testing on my local PC where PayPal sandbox couldn't complete the transaction, the referral worked fine and was recorded but when I've moved to a live server where PayPal can complete the transaction the referrals have stopped being recorded so I'm thinking that PayPal interferes with the User Referral cookie.

Do you have any ideas on how I could resolve this issue?