Waitrose currently have 3 for 2 on some pampers nappies. They also have an additional 20% off via 'pick your own offers' on baby dry sz 3 50 pack (they have other sizes with 20% off, but they're not part of the 3 for 2 offer). You have to have a mywaitrose card, but takes 1 min to sign up online.
That means £12.80 for 150 nappies, so 8.5p a nappy.
Or go onto the pampers website and print off 3 £1 off baby dry coupons (1 per pack) and get the deal for £9.80, so 6.5p a nappy.