Someone has sent me a pm suggesting the following
Bitterness in potatoes is generally associated with some exposure to cold. Cold results in the production of glycoalkaloids, as a defence mechanism, as they are denser than other compounds and can resist cold, even very close to frost, better. I do know that the U.K. had a brief warm period last month but has it been cooler since?
Duke......they were in the ground. :)