why have my runner beans failed this year

I think the problem can sometimes be crap beans in the packet. We grow climbing rather than runner beans. Mostly they are good, like yours, but on a couple of occaisions we have had no germination at all.
If beans germinate and then die off I would say it is the fault of the grower but when they don't germinate after using normal methods then I would blame the seed.

What I am doing this year and it has worked well in the past is to start the runner bean seeds in wet kitchen paper on a plate or tray on the kitchen window sill. When the roots appear I pot them up in small pots of compost and put them outside with wire netting over them initially to protect them from squirrels etc.Then when they develop into small plants I put them in my veg.plot beside their bamboo canes. If planting the seeds directly into the ground isn't working for you perhaps that's worth a try.

Always get the seeds started in pots or under cover. You can soon tell if seeds are no good if they don't sprout on kitchen towel or in vermiculite when placed in damp and warm conditions.
Once planted out, there should be few problems but Runner Beans need plenty of moisture at the roots. Here in central France they are not worth growing as the ground gets too dry unless you water them daily. However, with a wet cool autumn they produce lots of beans at a time when the French (climbing) beans have begun to wind up for the year.