Pay for your storage online.  Pay by credit card or Paypal.  You will need either your storage invoice number or your storage unit number.
 
Calculate how much by adding to your cart multiples of corresponding amounts (price x quantity) until you hit your invoice amount. I.e. if owing £84.29 add the following items to your cart Price £20 Quantity 4 (£80); Price £2 Quantity 2 (£4); Price £0.20 Quantity 1 (£0.20); Price £0.05 Quantity 1 (£0.05); Price £0.02 Quantity 2 (£0.04).
 
Alternatively you can set up a standing order either in your Paypal account or from your bank account.