Instructions:This was posted a couple of months ago, but still works at least until end of June, which surprised me, so I am posting it again (within rules I believe).
Either use it online, or they can accept it in store (at least did for me).
Just got £255 knocked off a Whyte T130. This bike is not discounted anywhere by more than £100 so it saved me £155 on next best price. If you are on the lookout for a new bike, this voucher is fantastic.