I think this is Gastrophysa polygoni http://www.naturespot.org.uk/species/gastrophysa-polygoni the abdomen is swollen because it's a female carrying eggs,
Thanks. Had no idea that a 'pregnant' beetle's abdomen would swell like that. Presumably most beetle sp. don't. Just out of shot was a large clump of Knotgrass which was also popular with caterpillars!