This is what you need to do:
How do I upgrade from forum only membership to full membership?
Please do not use the "join online" link on the website if you are already a forum member. Instead, please telephone Membership Administration on 0845 601 4231 between 9.00am and 5.00pm, Monday to Thursday. You will need a note of your forum membership number and to have your credit/debit card to hand.