To upgrade to Premium, login to the online dashboard, go to your Account tab and click 'Upgrade Now'.
You will then be directed to a Paypal screen, where you can select to pay with your Paypal account or with your debit/credit card. If you do choose to use a debit/credit card, make sure to use an email address that isn't connected to a Paypal account or it will not let you process the payment. The £29.99 payment will give you Premium membership for one whole year, and we will send you an email when your renewal is due. 

Once the transaction is complete, your Account tab will show that you are a Premium member and when your year of Premium membership expires. Sometimes Paypal transactions take a moment to process, but if after a few minutes you refresh the Account page and you still aren't Premium you can email for technical help. 

There are also ways to earn Premium for free!