This is a Royal Mail Box, known as a Bag Drop Box and it is used for the sole purpose of storing letters and general post for a postman/woman to top up their bag whilst on their round. This way they can ease the load by delivering the mail in batches, rather than lugging it all round in one back breaking go !
The cache is a magnetic nano, it's a type I've not seen too often, larger then the usual nano's, so I though I'd give it a go and see what the feedback was before I bought any more. You won't need tweezers for this one, but you will need a pen.
Thanks again to tonymusprime for giving me the nod to place one of these.