Never seen one before in my garden

Hello I have been a RSPB member for many years but have never used the chat before, but have just seen a bird in the garden and now I am doubting if it is possible?  I had my binoculars looking at a Song Thrush who was getting the blue berries from my Mahonia shrub, when I spotted another bird. It was about the same size as a robin, and very similar in shape but it was totally grey, and I mean proper grey with much darker grey head.  I called to my husband as I have never seen a bird like it before and he also saw it through his binoculars. Neither of us knew what it was so I looked in my rspb Pocket Wildlife of Britain book but couldn't find the bird.  My husband googled grey birds and the bird that looked identical was a Gray Catbird.  From the internet I can see that they have been seen in Cornwall on rare occasions, but we are in the West Midlands ?  Do you think it is possible?