If growing on a windowsill, you can help them get maximum light by cutting the top and one long side from a cardboard box and lining the inside of the box with aluminium foil. Place the seed tray in the box and have the open side towards the window. I start my tomatoes off every year by doing this with good results. If the room is warm, once the seeds have germinated, move the box to the windowsill in another room which doesn't get above about 15C and not below 5C for best results. As "A lad in" (new lamps for old, anyone?! 
) says, if they are kept too warm, or don't get enough light, the result will be tall weak seedlings every time.
