Some sort of vetch? Or lathyrus vernus?
Perhaps a picture would help?
It very much does sound like Common Vetch, it came up in my Mother in Laws garden this year probably from the bird feed mix she was putting out. I find them lovely plants.
Their only drawback is that they self-seed very energetically.
I am not sure but this might be purple vetch which is a native plant that grows in the hedgerow.depending on how you feel about wild flowers they can look lovely in the right environment or a menace if you have a garden that's shall we say organised.lovely thou.