What's certain is Bambers duties were not just limited to labouring duties.
He was a chargehand and gave orders.
Being Nevills son no doubt there was a degree or nepotism.
Not talking about Bamber.
Was asking how thousands of farmers worked in darkness. You gave an answer after 5 requests.
Torches and spotlights are ruled out.
The most practical is headlights or ambient light. Obviously if Bamber used a helmet light, he could use it to cycle. Otherwise he would use the ambient light.