It depends on where you are and how high up is your flat. The usual advice is not to put out anything tender before frosts end in about mid May but if your window sills face south and are high enough to be clear of ground frosts you may be OK to do it a week or two earlier. On the other hand, if you're in an exposed or windy site, best to wait.
If you have more plants than you need, you could try potting on the best seedlings into the window boxes but keep them indoors for now. Keep a few spares just in case and pot them into small pots to grow on and then give away the remaining seedlings or just put them on the compost heap.