photo'd on 1mm square graph paper, so about 5mm long.
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Looks to me like a ladybird larva. Couldn't say which species though.
I don't believe that this is the larva of a Harlequin Ladybird as the colouration for this species tends to be more orange as shown here: http://www.ispot.org.uk/node/143400?nav=related
Lat/Lng: 51.4, -0.2
OS grid ref: TQ3079