What's the diagnosis?

This rather dishevelled great tit has been around in the garden since the end of last year.  It flies without difficulty and behaves and feeds normally yet you can see it has missing feathers from the nape of its neck, disordered body feathers, discoloured wing feathers and distorted and discoloured tail feathers.  These photos were taken on 2nd January but I have seen it in the garden several times today.  It still behaves normally and looks the same as this although I didn't get follow up photos.

My first thought was that it had escaped from a predator but it has no sign of injury.  I think it must have some disease or infection to affect its feathers and I guess it must be fairly long standing, given the feather deformity and discolouration.  I didn’t get far finding the answer online.  If you google “damaged tail feathers” all you find is loads of stuff from budgie and cockatiel breeders!

I feel for this bird because this is how I look most of the time.  If you have any idea what it is please leave a comment.