On bur-reed so seems likely to be Donacia vulgaris but couldn't see the expected red/blue sheen down the middle of the elytra.
No interactions present.
Thanks for the information. I've found other photos labelled D. vulgaris that look similar to mine (though most are more colourful) but it also looks similar to photos of D. impressa, so I've added my agreement that it should just be identified as Donacia.
D. thalassina is another possibility. Impressa is usually maroon. But I've never seen thalassina or impressa on Typha. So, yes, best left at Donacia sp.
Lat/Lng: 51.4799, -0.2354
OS grid ref: TQ226771