I googled it and it looks like TRICHOMONOSIS which is sometimes called fat finch disease and for this poor bird it will probably be fatal,the BTO recommend the following
Rotate positions of feeders in the garden to prevent the build up of contamination in any one area of ground below the feeders. Empty and air dry any bird baths on a daily basis. You may wish to consider stopping feeding if you have an outbreak of the disease at your feeding station, in an attempt to force the birds to feed elsewhere at a lower density (although in reality they may end up visiting another feeding station and possibly one where no hygiene measures are in place.
I have removed all my feeders scrubbed and turned upside down the bird bath so it will dry,they will remain like that for the next two weeks,living next to Holywell dene almost everyone in the street has feeders etc out for the birds so in an attempt to help further i will print off a couple of photo,s and the above precautions and pass them around my neighbours (it may help).
Be vigilant and thanks for looking