I've been seeing this little guy in my garden for a while now and I've never been sure if the white patches are from leucism or some kind of vitamin deficiency. The white patches are pretty much all on one wing, and that wing has an issue where it won't fold in properly. It doesn't seem to affect its flying though. I'm a little concerned about it because I saw it again after a month or 2 today and it looks like the white patches are spreading, although it might've just been the lighting. Sorry for the photo quality, my phone camera is pretty bad.
It looks to me very much a leucistic jackdaw.
In my very limited experience, most birds with leucism do manage to get around and feed. If I'm wrong, I hope one or more of the more experienced forum members will correct me.
Also, a warm welcome to the RSPB community.
Mike
Flickr: Peak Rambler