Discounted Individual or family membership for RHS gardens

After booking the free tickets found on here I was given this offer.
1 years individual membership after checkout thats good for 1 adult and 1 family guest or 1 adult and 2 children for £20.99. Usual cost £55.99 with 30% direct debit discount, otherwise £79.99
Or, a joint membership thats good for 2 adults, 1 family guest and up to 4 children for £48.99. Usual cost £83.99 with 30% direct debit discount, otherwise £119.99.