Anaemic / Albino Sweetcorn seedlings

about 1/4 of my sweetcorn seedlings, now almost 2" tall, are white (well, slightly yellow).

Happened on the last batch too, about the same ratio, and they all died.

Any ideas?

Suttons Swift F1. Seed bought last year in the 50p-a-packet sale, packet says "Sow by 6/2010" - too old?

My Mini-pop sown at same time, alongside the Swift, have been fine.

Plant them out and feed them well.  They are very hungry feeders so plenty of good stuff at the bottom of their planting hole.