My local hide (Etherow country park, The Peak District) that i've been visiting on a regular basis for the past 3 years was once bustling with Chaffinchs, both male and female. They once seemed to dominate the area over other species of bird however we noticed 80% had what looked like fungal growth on their legs. Last year and again this year there seems to be very little numbers. I've seen 1 female there this year so far. I haven't seen a decline in any other bird in the area.
I've never had many Chaffinches in the garden but it's my perception that there are even fewer now, I noticed a male the other day with warty growths which I suspect is papillomavirus virus. We have a woodland hide on my local patch and someone commented to me that it was quite rare to see a Chaffinch under the feeders.
It will be interesting to see what the Big Garden Birdwatch shows up with respect to finch numbers in general.