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Well, it wouldn't be the first time that a British key has failed to work on the mainland! But I don't think it is arcuatum. If it is, there's a serious problem with our understanding of antennal ratios in this genus! If I've made an error here (and it wouldn't be the first time! :D ) then I'd agree that it could be cautum. Time to go back to the collection for some more study!
Thanks for your comment, Amaira - as you can see, impressions can be misleading! ;)