Any help would be most appreciated
The most generated errors being found on Opencart forum originates from contributed programming. The increased post counters are caused by redundancies of the same solutions that were already provided prior.
Try checking your cart page source code to see if you can find other elements that have the ID "postcode", and see if that leads you to the right module. You can then disable/remove it from that page, so it doesn't affect the normal shipping estimator.