Hi Jaqui,
I think, once again, dealing with runner beans may be down to context. I plant my runner beans on the same patch each year and have lived in that bed quite happily for the last 10 years.
I always cut the plants off at ground level and then mulch.
Though I always raise enough plants each year for a complete planting out, my seedlings stay in root trainers until the plants from the previous year have sprouted as I prefer to use these rather than new seedlings. I sometimes have to grub around to find the plants which have overwintered successfully.
This year my overwintered plants (sown in 2017) produced a far superior crop to the (2018) seed raised plants, the latter being dreadfully poor. My conclusion is that the plants from last year had an extensive root network which was much more developed and robust than the new plants. I will also stick my neck out and guess that the roots lay dormant and did not rot, so had a deeper root run, so were not so susceptible to drought as per 2018.
Alys Fowler suggests thet may be kept for 2 years if there is a mild Winter and believes that they give a reduced crop in the second year. https://www.theguardian.com/lifeandstyle/2010/aug/28/alys-fowler-runner-beans
Now, the proposed method for keeping the tubers over winter seems to be to lift and store as one would with dahlia tubers as they can either freeze or rot off or mulch and keep them cozy in the ground. So the experiment should be as follows:
Mark where the plants are so they can be investigated in May when they should be popping up.
Cover in fleece to protect any early shoots from late frost (and some straw may help under the fleece too), and
Do no-dig gardening to build up the humus so the ground drains well and does not rot the tubers….
