I read about that deal over on HotUK deals website.
It sounds to me that it's an email discounted sports offer sent to selective limited number of nowtv customers only.
Not eveybody would get that offer and for the record I don't get the offer myself
You could try live chat and see if they can offer you a discounted month sports pass (I doubt they would offer you for that price mind).
If you wan't to comit for 3 months then nowtv have a £20 per month offer on their website for new and existing customers.
I know it's not as attractive compared to £13 one month sports offer you have seen, but still a saving against the normal £33.99 monthly price.