Instructions:Howdy, first time poster, long time lurker, so be nice.
The voucher does exactly what it says on the tin. Not sure what the minimum order amount is, although I ordered something the other day worth £30 prior to the voucher being discounted, and it worked (if someone finds out the minimum spend, then just edit the post or something).
Be warned it's for first time orders only, if you create new accounts I'm not sure how likely they are to cotton on due to their weird way of discounting the vouchers (take the full amount from your card, then credit it with the £18.50), so it's your responsibility if you get found out etc. Hope this is of some help to someone.