Add This To My Cart Not Proceeding

[mc12345678]

Thanks for the rapid responses. So, simply upgrading to 1.5.4 should resolve this, do I understand that correctly?

Based on the information provided thus far yes, that would be the expectation. (Ie. If nothing else was changed on the site by you or others with access to the files then considering the age of the software and known issues, worst case is that you have an updated version that still has the same issue, but at least less issues than before.)