the secrets to great yorkshires is to have the batter well rested & at room temperature, the oil & tray smoking hot (literally to the point where it would melt your face if it spat at you) and most importantly is NOT OPENING THE DOOR until they are ready (and if you do, close it gently, if you bang it shut you'll knock the air out of them and make them sink) .
the other must is a dirty tray. buy a yorkshire tray. cherish it, love it, but never ever clean it. give it a wipe when its dirty but if you scrub it you will never get it back to where it was. the batter needs rough edges on the tray to help it climb!
oven at 200 for the initial cooking then dropped to 180 to finish them off for the last 3rd of the cooking time.