If this year's Stirling Prize judges had been just and true, unswayed by famous names, political correctness, fashion-consciousness and urban bias, then this was the building that should have won British architecture's top award. It didn't, which came as no great surprise. But let's not allow the mere outcome of such a seductive and successful media-led event to divert us. Instead, consider this. Why is the Downland Gridshell, by Edward Cullinan Architects, the most significant new building in Britain in 2002?