I've just done a very rough calculation and come out at a figure of around £2000 for just materials.It would probably take a couple of guys 2 days to get it done,that's if the ground has been cleared and the hardcore is in and level. Add in transport costs for everything,drainage,shuttering et al,and if the quote comes in under £4000 then I'd be happy with that
150mm seems a bit excessive unless you're driving tractors over it,with the re-inforcing steels,you could go to 100mm and save a bit of cash,but not much....Ree