Hopefully this isn't a repost, did a search but couldn't find anything.
Just back from B&Q in Newtownabbey, NI and they have an offer on alot of concrete products. Basically if you buy 5 or more you get 25% off. I'd assume this is nationwide.
For anyone building a fence they have Hanson postfix @ £4.68 per bag (Homebase £4.99 per bag), so if you buy 5 or more they work out at £3.51 per bag.
Anyone who has used this stuff before will know how good it is and easy to use.