Your order number can be found in your account under orders or in the order confirmation email
Log into your Just Geek account and head to the dashboard. Once in your account in the dashboard section you will have a table containing all your orders, on the left look for the column named 'Order' in this column you will find your order number.
The order number can be found just below the shipping address and above the section showing payment method. It can also be found in the email title of the order confirmation.