Well Beltraide were very helpful and are looking into it. They weren't aware that residents could bring in household goods duty free. I'll speak to them again on Monday. Very nice staff there and very attentive.
In the meantime, I thought I'd better arm myself with some ammunition, so I downloaded some very useful stuff from www.customs.gov.bz/duty.html
I was impressed that this information was so readily available on the internet.