I'm sure I should be able to identify this one, it's so distinctive, but I'm having no joy. Was originally thinking along the lines of Tussock / Vapourer. It's probably terribly obvious, but I would appreciate some help! Thanks.....
Thanks for that Clive. I assumed it was in the Lymantriidae somewhere. I wasn't totally convinced by the white satin though; it seems to have a rather distinctive stripe down the back, whereas the one I posted is approaching the 'tufty' appearance of a vapourer with quite a lot of black hairs. I do see the white heart-shaped blotches though.... I suppose there is a certain degree of variation with all these things, particularly in terms of maturity?
I can see why Clive suggested White satin, but it is definitely Acronicta rumicis, the Knot grass moth (a Noctuid). The downward facing white Vs separated by red marks along the sides are diagnostic. See http://ukmoths.org.uk/show.php?id=795
It feeds on a variety of plants, including Polygonum aviculare, Common knot grass. Yours appears to be on a sp. of Polygonum.