Well, they say it's free but you have to spend £40 or more on another item to get it (which I don't have at the moment) so I thought I'd share the code in case someone else was planning on buying something.
I think the deal is you pick an item/items that you want to buy, choose either one of the the "smart vest" or "essential vest" to go with it, and then enter the code "Birthday1" at the checkout and the vest is free.
I really like their stuff but am saving up for a holiday at the mo so just can't justify adding to my wardrobe 
Happy Shopping 