If getting less yield doesn't bother you leave them be.

How are they going to get pollinated in the polytunnel?

Like most plants French beans need lengthening day light to grow. I'm not sure why you planted in September but I'd yank out and start again March time. There are no natural pollinators around at this time of year.

I know that French beans or any bean plants for that matter are self pollinating. That would go for peas and tomatoes too. They do not depend upon external pollenators unless one would like to cross pollinate in the fashion of Gregor Mendel.

That's true I was thinking of runner beans Loosestrife but I still doubt they will grow well having been planted in autumn.

I would say that MG's lack of daylight, and the cold and damp will finish them off. Even if they begin to set fruit I think you will find that they will rot off.

Right, planting as the days grow shorter will certainly give diminishing returns to say the least. It would be better to call this current planting a fail and start anew. A poly tunnel is not a hothouse with supplemental lighting.