I contacted the RSPB because I couldn't believe what I was seeing and I have just had a reply back. Apparently, coots eat small insects, snails, voles, mice and even other birds eggs! So I am glad I confirmed what I thought was happening. You can see the little voles ear in this shot
I have watched these quite a lot, and often look at what they bring up to eat when they dive, in the hope of an interesting shot, but I have never seen them with anything like that. Great find, Rosie.