That the little naughty things eating the bean and pea plants are actually the aptly named 'pea and bean weevil' and not flea beetle as previously mentioned. Looked up the little blighters in the Organic Gardening Catalogue yesterday.
As suspected the weevils affect early sowings the worst whereas later ones grow faster and don't mind being nibbled as much. The book also recommended that to discourage them you shouldn't grow vetch as a green manure...bah! Anyway I lay the mis-identification blame on my allotment friend who was very confident they were flea beetle ;0)