I would have thought it is too early but really it depends on the weather. Has it been very hot and high humidity for a few days, night as well as day above 10C? There is a farmers blightwatch site you could check - sorry I don't know the URL
Yellow leaves could be various other things - take off any you see and pop them straight into a bag so minimum chance of spreading spores if there are any, then dispose of them away from the potatoes. I have had whole plants collapse from something other than blight, so you may be ok. Also only water from below, so the leaves are not wet. Fingers crossed it's not blight