I do work for tarmac from time to time.
I had a word with the manager there and as long as your buying a pallet at a time it works out very cheap.
I needed a load of white cement and got it for £7 a bag rather than £12 out of the merchants.
Yeah I looked into them. It works out at £3 per bag but that doesn't include shipping. By the time you add shipping it's £4 roughly or a little less. Hardly worth me storing them as I already only pay £4.50
Maybe if I buy x2 pallets they'll ship free but dunno. Don't know what shipping costs yet. Have emailed them.