Does anyone know what species this is?

Hello everyone!

I'm sorry for the upsetting pictures, but this poor soul flew into our window earlier and I was wondering if anyone can help identify the species.  The beak is very large and finch-like and it has a black cap (although this could I guess be juvenile plumage), but there is a brilliant white flash/block to the area around the base of the tail, which I don't recall seeing on our other garden finches before.