Hi @GaryFoster, I checked the link where the html button is: http://thenugs.co.uk/the-nugs-t-shirt/

I could not reproduce the error and can't see the variables passed as you are using a hosted button, which code is on PayPal server and not visible to others than your profile. This error is usyally returned when for example the shipping address is not passed correctly or some of the shipping variables are missing or empty value. But to be sure we will need to check the hosted button variables, eventually I would suggest to open a ticket to MTS: MTS then at the bottom click on "Contact Support"