"He explains that there is a stained-glass window in St Andrew's Church, just down the street, that contains a clear metaphorical allusion to the Holy Grail, and a cryptic hint that it might be hidden in Hertford. In the picture, Acheson adds, Jesus and Mary Magdalene are looking at each other 'in a very meaningful way'. (Later, I find the window, interrupting local parishioners who are decorating the church for Christmas. I think I can see what Acheson means about Jesus's expression, although mainly he just looks a bit depressed.)"
While on the topic of Mary and folklore about her, France is littered with stained glass showing her in Provence, where she is believed to have taught and lived post-Jesus. Here's a window from Chartres.