Standard price is £17.93 which is joint cheapest with anywhere on the internet at the moment to the best of my knowledge but using one of the following methods it can be had even cheaper:
Using the 5% valentines Offer gets it down to £17.03 (5% off)
There's also a 10% discount link but technically you should be a Halifax Ultimate Reward Current Account holder. (£16.14)
Quidco is 10% and making it effectively £16.14, assuming you can't stack quidco with either of the above discounts.