Grey Dagger Moth Caterpillar
This exotic/scary looking wee beastie is a Grey Dagger Moth caterpillar.
I spotted several of these on an apple tree in my garden. I had never seen anything like it before, but a quick search on the internet identified them.
It seems they don't do too much harm to the tree, and the adults may actually help in pollination - so I just took a pic and left them where they were.