Hopefully this works for other people as well.
I bought Iron man on blu ray yesterday but only had a £2 voucher, so I added 2 copies into my basket and put in the code so it took £2 off one of the films, I then placed my order and cancelled the full price one. They sent Iron man blu ray today for £5.99! I have only been charged £5.99 from my play trade credit.....Bargain! I have not got any more vouchers to test this with, but if you have £5 ones you might be able to really go to town and get items for free :)