Checkout not clearing previous parts

Hi everyone, using Opencart 2.0.2.0 and Journal 2 version 2.16.7 - I have an issue where the checkout does not clear - basically if I add an item to the cart and go to checkout it displays everything perfectly - I can select my payment method and complete the transaction without issue - however - if I then go back onto the store the and add a new item to the basket and checkout the previous item is still in the checkout and can not be deleted. It also will only allow checkout through the previously selected method. This is something that has worked perfectly for a couple of years and now all of a sudden it has this issue. It makes no difference which browser I am in. Also if I clear the cache it does nothing. If I leave the site for more than about 10 minutes it seems to finally clear and be useable again. It also appears to stay in across platforms and other peoples sessions.

It is only the items in the checkout that stay and the selected payment method, all other things clear every time. The website is:

www.regalautosport.com/shop

Anybody have any ideas on what could be causing this to happen?

Using old OC version.