To identify birdsong, go to the RSPB website (www.rspb.org.uk) where you can listen to all the common British birds, to help you identify it. (click on "birds and wildlife" then "birds by name" and you get an A to Z list)
As for goldfinches and nyger seed. We started feeding with this just over a year ago and in that time, I have never seen anything else feed on it except goldfinches. We started out with an occasional appearance from one goldfinch for a few weeks- we now have regular groups of up to 15 birds feeding from the two nyger feeders and the seeds dropped on the gound. Persevere, it'll be worth it.