I had the same problem with coriander. I tried growing some from seed on a window sill and also some in the greenhouse. The indoor ones came through more quickly then were really leggy whilst the greenhouse ones were shorter and stockier. I put the indoor ones into the greenhouse then killed them the next day when I forgot to open the greenhouse up before leaving for work!
My basil in the greenhouse looks really healthy - any chance of putting them out in a greenhouse or coldframe until they've grown on a bit?