I have not seen anything like this before and I have no idea what is wrong with it. I don't think it is caused by any external parasites as I would expect it to try and preen more than it seems to. Something is presumably affecting its central nervous system but what??? Could it be caused by some sort of virus or has it collided with something and suffered some sort of brain injury??? I simply don't know what is causing this abnormal behaviour at this stage.
The bird is ringed and it is probably one I have ringed but I doubt it had this condition at the time of ringing as I am sure it would have been obvious. Also I haven't caught a Chaffinch in the garden since early December and I would have noticed a bird displaying this behaviour sooner if it had been affected for that long.
I will make a few enquiries and do a bit more research to see if I can find out what has caused this. I will also keep a record of subsequent sightings of this bird to see how long it survives with or exhibits this condition. I just hope it isn't caused by a disease that can spread to other birds. If anything comes from this I will keep you posted.