The annual subscription of £20 is due each April, or upon joining if part way through the subscription year.
If you have established a recurring PayPal payment of £21 (which includes the PayPal charges), renewal will be on the anniversary date established when the payment was set up.
Most members, after joining and paying the first year subscription by bank transfer, set up a standing order. The form for that is here : SOA SORD FORM 2017-18
If you wish to renew your subscription by PayPal, the links are below.
PAY SUBSCRIPTION BY PAYPAL :