Wren (or a Dunnock)?

I was trying out some of my husband's lenses this morning taking pictures of birds on the bird table and feeders. A variety of birds were present, but this Wren was not the subject, it was in the background, just at the edge of the frame, so it is not as sharply in focus or as well exposed as it might have been; the picture has been heavily cropped.

Update - 1425 hours - According to iSPot this is a Dunnock and not a Wren, all I can say is that this was very small for a Dunnock.