Of course it's also possible (and in same instances is clearly and statedly the case)that the fragments were the gift of 19th century antiquarians who purchased them from curiosity shops in Anyoldwhere. Obviously I like to think that some at least of the survivals are in the buildings they were made for.
Anyway, here's an excellent example from Kemsing- an almost complete roundel showing the Mother of God with the infant Jesus.
Kemsing also has this. I think what we're seeing is John the Baptist in hairy shirt pointing out Jesus as the Lamb of God. I love the trees.