Stringy Scarlet Emperors.

My Scarlet Emperor runner beans have produced tons of pods but the ones we've just eaten were disappointingly stringy.  I realise that in the case of some of them it may be because I've left them too long before picking.  I've grown them before without a problem but this is the first time on an allotment plot.  It might be I haven't got over there to pick them often enough.  They are growing right next to broad beans.  Could they cross pollinate and could that produce stringiness?

Thanks Lesley.  Of course I picked four pounds of the so-and-sos yesterday and I'll now have to sort the old from the young  before freezing so I'm not freezing stringy ones.  That'll give me something to do whilst watching rubbish on TV tonight!

Thanks again.  I'll pick more often in future.