My potatoes dissolve when cooked ! help

Please does anyone know what I am doing wrong?
This is the second year this has happened. I dug up my King Edward potatoes last week they were planted in April, they look fine, a bit small maybe. I cut them up put them in pan of water and within a minute they are turned to watery mush, very dissapointing.
I have clay soil which has had loads of horse manure dug in, I watered and did give them bone meal too. any advice would be welcome, thank you.