I just happened upon a product called Accoya. Seems to be (basically) a way of preserving timber by treating it with acetic anhydride. (Yes - noticed on the Grand Designs Live site.)
Must say, the claims are quite impressive. E.g. potential 50+ year life as a window frame. Set me thinking, would it be possible to do anything like this myself? I doubt I'd want to be dealing with acetic anhydride (and re-drying it after use might not be easy) - so any ideas? I guess blagging some non-brewed condiment from the chippy and soaking some wood in that isn't likely to be sufficient.