I have a pile of wood bark chips in the corner of my garden - scraped up after previous owner had some the garden chipped over for their kids and jungle gym. I'd guesstimate enough for 18-25 m2 coverage. Free to a good home.
Could have sorted you a boot full of wood chippings if I'd known yesterday, brother in law has sheds load (yes that how much he has, about 3 sheds full), won't be seeing him again now until August bank holiday though. Unless your over Chelmsford way yourself anytime soon?