To quote Jamie Oliver, (sorry but like him or loathe him he is a top chef) who is the reason i finally found the secret to perfect Yorkshires after 10 years trying, you should leave the batter to rest for a few hours, to allow air to be absorbed into the mixture, resulting in lighter Yorkshires. If you are making the batter up at short notice you can get round this by weighing your flour quantity carefully as usual, then adding about half a tablespoon extra of flour to the mixture.