I have the same problem as you Kate, all the seedlings on my window ledges always grow tall and leggy with very few leaves. I have an amazing array of sweet pea seedlings on my bathroom window sill at the moment that are 5" tall with not a leaf in sight!
Have come to the conclusion this isn't a very good place to grow seedlings. I can either wait until its warmer and try growing some in a slightly sheltered place on my balcony, (I live in a flat). Or I can buy them as small plants from the garden centre and this idea is becoming more and more attractive to me.