Local knowledge will help us. Several Wellington businesses have PO Box nos. The boxes will be at the sorting/delivery office, which is known locally to be at the back of the old Post Office. Customers will use the front entrance. The Royal Mail postcode finder for PO Box 108, Wellington, gives TA21 1BE - as ODC found - and the map shows this adddress to be at the old post office. Since Box 108 is a Return Address, it seems likely that organisations post their own mail with this on the back, and any returned envelopes are collected by some organization in Wellington to be dealt with however. Interesting.