Please refer to the 2017 pricing here and take the retail price divided by 1.2 as mentioned above by @Lostinlondon
I'll explain once and for all the reason for dividing it by 1.2. All prices are inclusive of VAT which is 20%. In short the retail price is 120%. To work out the duty free price, simply divide retail price by 1.2. (There might be rounding up to the next closest £.)
That's very helpful! Thank you so much!!