Try phoning Wickes, Jewsons and a local garden centre. See what prices they come back with. It rather depends you are buying, and the colour of the blocks.
My paver hubby says try a builders merchants, such as Buildbase or your local one and you will probably find they will be a better price. Also try ebay as Marshalls sell off blocks on there. The price will depend on the amount you need. Try bartering between different builders merchants as we know many people who have managed to push down the price.