I had no idea what this was when I saw it in my garden today. I've since found out that it is a Robberfly.
They are quite an interesting group of insects. There are 27 British species. All feed on other insects, which they usually catch in mid-air. The proboscis is used to pierce and suck the juices from its prey, and it has dense bristles on it's face to protect it's eyes from struggling prey.
This one might be Common Awl Robberfly, Machimus cingulatus.