Broad beans do cross like runner beans, so in an allotment situation you will have a number of crossed beans. If you have left them to mature before shelling, then they would be viable, but you will have your own F1s. There is another issue about only using late beans, the earlier ones having been picked and eaten. This selects your beans for lateness. For seed saving you would aim for early as well as late pods, Lez.
With those considerations in mind, try the seeds, but in addition to normal seeds and compare their performance in your situation. Like TeeGee said, don't rely on them as the only seed for next year.