I sent an email with a link to my previous post and the videos to the Garden BirdWatch Team at the BTO and they kindly forwarded it to the vets at the Zoological Society of London. The vet couldn't give a diagnosis from the video clips but he did say that it is exhibiting signs of motor neurone dysfunction without impaired appetite. There is also what looks like an incident of weakness, with momentary collapse and that this could be an end-stage disseminated bacterial, viral illness or migrating nematode or protozoan parasitism. He also said it is very unlikely to be trichomonosis primarily but could be a secondary complication, or indeed something completely unrelated.
Well that shows there are a few possible causes and that it certainly doesn't look good for the Chaffinch. However it was still around yesterday so there is a small chance it could still be alive, but even if it is it probably won't be for much longer. So while the outlook for the Chaffinch is bleak it has encouraged me to find out more about the Garden Wildlife Health project and how to report incidents of sick and dead birds along with other garden wildlife. You can find out more about the Garden Wildlife Health project by clicking here and the BTO Garden BirdWatch by clicking here.