Maincrop potatoes yellowing

I planted some Maincrop seed potatoes on 8th May, and gave everything a good water before I went on holiday last week.
I've just returned today and am very pleasantly surprised to see how good everything is looking considering how hot and dry it has been here, except my Maincrop potatoes which are falling over and yellowing - more so than the earlies!
I planted the earlies slightly earlier, about a week or two at most, and the other difference is that the maincrops are in the ground, whereas the earlies are all in raised beds.
Do you think the yellowing and dying of the Maincrops are more to do with a watering or other issue, rather than them being ready?
I could of course just dig one up and have a look... One of them is belle de fontenay which I understand to be a small sized potato, so would they perhaps be ready earlier than other maincrops?
Many thanks as ever! :-)
Posts
more likely to do with watering, now is the time that the tubers are swelling, so the plants have put their full water supply into growing them instead of supporting themselves. problem is I don't think its recoverable situation as watering won't help the plants, though it might help the forming tubers
Thanks treehugger80, it is a shame, but I dont think that I will be short of potatoes this year, so not the end of the world.
I checked yesterday, and the Harlequins are fine, the Maris Peers are kind of half and half, and the Belle de Fontenay are all dying. I will give it a week or so before digging some up to see how they're looking.