Why no TU banners at the Tate?

Have you ever seen one of those front gardens were people have started with a few garden gnomes, cemented in some sea shells and gone on to create unbelievable, and some would say unbelievably naff, fantasy grottoes? This is the kind of unsung masterpiece celebrated at Tate Britain's exhibition of British Folk Art that brings together old shop signs, pieces of sculpture fashioned from discarded chicken bones, ships figureheads, tapestry and all sorts of odds and ends produced by the untrained and unknown. One glaring omission was the complete absence of any mention of what I have always considered to be the epitome of British "folk art", the trade union banner.