A rather faded cinnabar moth, Tyria jacobaeae (Lepidoptera: Arctiidae). Normally these are as brightly coloured as the 6-spot burnet moths; this is probably an old specimen but to be old in early May, like this one was, suggests it might have over-wintered as an adult. I don't know how likely this is.
The caterpillars look like this and they were very common on ragwort (Senecio jacobaea) this year. See here for one fending off an ichneumon wasp.
Photos taken in the field below Chazey Wood, near Caversham, UK, on 2011-05-04 and 2011-06-26, respectively.