Don't have a window feeder recommendation, but the RSPB sell a feeder mix which is great. Stuck it in the feeders in my parents back garden, and they are now routinely visited by goldfinches, bullfinches, black caps and also a sparrowhawk due to the increased bird population.
On the talk of garden birds, any suggestions on a window bird feeder and, more importantly, what to put in it. This kind of thing I guess:
I've got some fatball holder thing but it doesn't seem to have attracted any birds yet.