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If you decide to go for sleepers use proper creosoted railway ones if you want them to last 5 minutes. Retain with posts 18"+ into the ground with dry mix 4:1 concrete whacked in hard around each post. Scaffold boards will only last a couple of years, but if you concrete the posts in as above are easily replaced. New sleepers are a bit crap but easily slung on your shoulder, railway sleepers can be handled by one person if one end is strapped to a sack barrow.
As someone used to the finer tools in life I’m going in with a brush cutter and long arm hedge cutter