We had a similar thing happen with what looks like a similar type of operation Steve (large community arts centre). They did it in a slightly different way, that is every person we spoke to directed us to someone else. When we found the person responsible for payment they tried the 'Your quote wasn't in the correct form' thing. IIRC our singer just persisted, went to everyone about it (local council etc), and after a couple of months we got paid. What really annoyed us was the the gig was rammed, with the businesses in the space itself (food, bars, retail) telling us they'd never had such a large crowd in, in fact one of them ran out of food, but the people who run the place are just creating bad PR for those businesses by ensuring that bands like our will never play there again 😡
Hope you get it sorted Steve