Cart quantity can't be deleted or changed

-- url of the page with the problem -- : www.coasttocountryweddings.com.au/index.php/shop-online-at-ctcw
-- HikaShop version -- : 3.5.1 Business
-- Joomla version -- : 3.8.12
-- PHP version -- : 7.1.21
-- Browser(s) name and version -- : Firefox 61.0.2

I've seen a few forum posts on the same topic but cannot resolve for my site. I have items in the cart and I'm not able to delete cart contents. When looking at My Shopping Cart, I can use the + / - symbols to go down to qty 1 but they immediately refresh back to the original quantity. I can't delete from the cart using the X button, nor change the quantity. When I'm at Checkout and the in the display of the cart contents, if I change the number in the box and hit the refresh button, the entire Cart disappears from the screen. Could you please test using the URL I've supplied and let me know if you're able to help me resolve it. Thankyou.

Hello,

Your issue is probably related to a JavaScript issue caused by your template, can you test it with one of Joomla default templates ? Thank you !

Kind regards,
Mohamed Thelji.

I have tested with a Joomla template and the issue is still occurring, I can't reduce quantity or remove items from the Cart.
On further testing : I've changed PHP down to v 7.0.31 because we were also experiencing problems with reCaptcha on forms, now fixed. The problem with Hikashop seems to be with the Cart. If I add items to the Cart and go straight to Checkout, I can add more and use the refresh, I can delete the items. But if I go back to 'Cart' then it has only one product displayed even though I may have subsequently added more products, and I can't change the quantity of the product that's in the cart, nor delete the product.