It was from O2 and since I had not ordered or requested anything I assumed a mail-shot. Interestingly though this was not your usual envelope or cardboard flyer. This was a heavy (for its size) tangible rectangle. Addressed to me and labelled with only the blurb "Everyone Loves Surprises". More Curious.
Upon opening said tangible rectangle I discover it contains a chocolate bar I've inspected it, smelt it and tasted it - it's a real live chocolate bar! Curious and happy now!
It turns out that O2 are rewarding customers who signed up to their "Rewards" scheme by sending out chocolate. Not one to complain I will happily consume 'reward' and accept occasional marketing SMS and emails on the basis I might get another bar of chocolate in a few months time