I would recommend proper old railway sleepers, they're the only thing that will last long. They're a bastard to work with and move around. Most of the cheap oak sleepers are not native oak and don't last much longer than softwood.
Joys of not being in London, I can get English oak sleepers (proper ones) for £25+vat. The issue I have with railway sleepers (have used them in my front garden) is when they get hot, 4 year old sits on them, and she ends up with creosote on her clothes...
I would recommend proper old railway sleepers, they're the only thing that will last long. They're a bastard to work with and move around. Most of the cheap oak sleepers are not native oak and don't last much longer than softwood.