I didn't dig a trench but plenty of organic material was dug in and fertiliser. So far I have harvested just one bean . There are some others but in general it is a poor show. As I said, plenty of flowers which look good, and the plants look healthy enough. I shall try the trench method nex year. As a thought, the soil is acidic here so maybe it needs lime?
If you have plenty of flower the issue may well be pollination. This year I have been banging on to anyone who will listen about the huge reduction in the number of flying insects around the garden, allotment and house. I'm thinking of all flying insects and not just bees and wasps. We have only seen butterflies in any number in the last two days. I've had ONE cabbage white on the allotment. I've yet to see a moth this year. It's my belief this is related to pollution, chemical usage, climate change etc. and is a very worrying sign. There are serious scientific reports on this issue not just my observation.
In the case of your beans if there are no pollinating insects around there will be no pollination and therefore no beans. Runner beans are usually pollinated by bees, no bees equals no pollination. I guess other insects will help with pollination but it is the bee you need.
This year our garden and allotment bee population is way down on the average. Last year it was significantly higher because we had a tree bumble bee nest on the allotment. I made a couple of areas potentially attractive to them but none arrived. We have insect friendly plants in the garden and I've dotted bee friendly plants around my allotment. Frankly it's not making much difference.