NETQUOTEVAR:ERROR at checkout page

[has]

When i add an item to the basket and then go to the checkout page, i get the following error NETQUOTEVAR:ERROR. Then when i try to go to the next page i cant because their are fields on this page which need to be filled, but they are not there. Website address is plumbspares.biz. Any help would be appreciated.

P.S I searched for the above error here but theres no info about the "next" button not working.

Thanks,

Has