I've also heard it recommended that you heavily mulch the plants over winter to protect the roots from frost. So last week, hearing we'd got snow on the way, I went out to mulch them with a good, thick layer of straw.
And then we wait!
I've collected seeds from each plant and have put them into labelled envelopes so I know which seeds come from which plant.
If any of you have tried this before or know of any other types of bean that can be grown perennially, it would be great to hear about it!