Hi Christopher. I've seen other examples of this one unanswered on the forum and had been struggling with it until recently.... I was looking in our UK moth collections here in the Centre for UK Biodiversity at the NHM to check a suspected Fox Moth finding..... and there it was, a fine example of our velvety black and yellow 'caterpillar'. Therefore I can be pretty certain that this is an early instar larva of Macrothylacia rubi, Fox Moth.
The mature caterpillar is quite different (that's why I was confused) and thickly covered with blackish and tawny-coloured hairs and found feeding on heather (Calluna) and bramble (Rubus), amongst other things.
Yours very happily.... Gill