Instructions:First find the currnet Metro style CREDIT offer (£5 free credit at the moment).
Then use this once-per-account 50% offer to essentially double the credit to £10.
If you buy a 9.99 item, using the blinkbox50 voucher first, then the rest gets taken from the free £5 credit (if you have it).
If you buy a £4.99 movie, and use blinkbox50 first, then it costs you £2.50, leaving you with £2.50.
You will find that you cannot use the voucher again in your new account, once used it is invalid for any subsequent purchase by that account.
Just to confirm that you can use it once per existing account as well.