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Oops. I've misspellt sericea.
Thanks for the comments, any idea what the critical features are that shoe the difference between Plateumaris sericiea / discolor- just in case I see it again!
The differences between sericea and discolor are fairly subtle. On the top of the pronotum, discolor has "punctuation distinctly stronger in mid-line, obliquely wrinkled throughout" while sericea has "punctuation equal and fine, smoother, without oblique wrinkles".
There are also differences in lengths of the antennal segments (but difficult to appreciate without comparing the two species side by side), and differences in the male genitalia.
This is taken from:
Cox, M.L., and Menzies, I.S. 1996. Notes on the natural history, distribution and identification of British reed beetles, Br.J.Ent.Nat.Hist. 9:137-162.
Available from BENHS for £4:
P.S. see also this informative blog post from Dave Hubble:
Entomologist and biological recorder
Lat/Lng: 52.1, -3.4
OS grid ref: SO0451