The very hungry caterpillar

I almost didn't spot this little eating machine yesterday teatime until I looked more closely, but then, catkins aren't usually quite so stripey or hairy.  To my delight (for more photographs) the caterpillar hadn't moved more than a few twigs further away by lunchtime today, and was still munching away on my silver birch.  Fortunately my tree is rather too big for this caterpillar's little tummy.

Despite trawling my big book of butterflies and moths, and trawling the internet afterwards, I haven't managed to positively identify it yet, even though its shape and markings are familiar.