Martin wrote:
Being organic should rule out using old railway sleepers, which are
saturated in nasty chemicals.

There's that indeed. Heard of a story by a friend whose friends had
bought lots of sleepers to reconstruct their dining room ceilling. They
were very 'on hand' people and did it themselves. I don't know where
they got the sleepers from, but wihin a few days they wondered what the
smell was. Yep ... those sleepers smelt of pooh.