I think some kind of vetch too. Jacob's ladder (Polemonium) has bluer, at open flowers, vetch has pea-type flowers (and seed pods like little pea pods though I can't see any of those in the pictures).
Doncaster, South Yorkshire. Soil type: sandy, well-drained
Thanks for all your suggestions. I thought it was a vetch but I have been told that it was bought from a garden centre. I will check for seed pods when I next see it.
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I wonder if it is Indigofera
Vicia sepium have tendrils at the end of the leaves...no seeing these in pics.