Help to identify a bird

Hello! We were walking in the woodlands of Durris (Northeast Scotland) recently and saw a tiny brownish-grey bird, smaller than a tit, with a round blue and dark grey 'eye' shaped marking on its wing, similar to that seen on a butterfly's wing. It was flitting around in amongst the trees, close to the ground. It was daytime.

Can anyone help us to identify this bird? We've been looking for ages and can't seem to find any information on it.

Sorry but it was too quick to get a picture!

Thanks in advance!