Peter Brears has a long acquaintance with jellies in every guise. He was fed them in childhood, he turned to curating their moulds and associated artefacts while director of York and Leeds museums, he has made them for innumerable historical food shows and events.And jelly is a much bigger thing than some packet from the supermarket mixed with boiling water. In the first place, it was not factory-made gelatine that did the setting, but any number...