http://en.wikipedia.org/wiki/Silverfish
I know they live in damp conditions, as a builder I often find them in old houses along with their grubbier relatives called firebrats.
Just checked a bit more. It is a relative of the silverfish called a bristletail.
Sweet thanks! Thought it might be a kind of silverfish but I never knew they lived by the sea... The more you know :)
So I was rock climbing on a beach in Pembrokeshire, Wales, UK today. I put my hand in a crack to pull myself up and out crawl loads of these horrid things... Can someone tell me what it is?