You could e mail the BTO and ask for details of any ringers working with Brambling then contact the ringers directly. Some ringers can be quite secretive with their data so bear that in mind! I'm sure there must be a group or too working on this in the UK.
May be this one could help you. Payevsky, V.A. & Shapoval, A.P. (2000) Age structure of Passerine populations according to ringing results. Avian Ecology and Behaviour, 4, 55-66.