About 6-7mm long.
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Do you know what plant you found it on? There's one a bit like this that feeds on Viburnum leaves.
It was on my window, however there is a viburnum (Guelder rose) a few feet away. And I've just remembered, I spotted a larva in June this year on that plant: http://www.ispot.org.uk/node/272157?nav=users_observations
Glad to be able to help. "Pyrrhalta viburni - Viburnum Leaf Beetle" is very probably the ID, if you want to add it. I must get round to uploading my photos of this species!
Thanks - will add id.
Lat/Lng: 51.5, -1.8
OS grid ref: SU1987