Harvesting onions and potatoes

I’d appreciate any advice on harvesting my onions and potatoes. Spuds are Duke of York, and they have profuse foliage but few blossoms. (I have just read they are prone to blight! Ick!)Onions are poking their “shoulders” up above the surface, and looking very healthy – they are Red Baron and Sturon. The books say to wait til the onion foliage dies down, but a few have already produced seed heads, which I have snapped off.