Hi,

I have also planted giant sun flowers around the same time and morning glories etc and 6" sounds fine to me. Are you having them on a window sill that is over a heater? Giant sun flowers grows very quickly, you can go away and come back during the day and it has grown. Lucky you with south facing window. How close together are your sunflowers? How many in a seed pot?
Also marigold and petunia, once germinated are all fast growing plants, especially since the weather is warming up.
Don't more seedlings until you get a second pair of leaves.
I am wainting another week before potting my sunflower into individual pot and growing them on. Then probably a couple more weeks before putting them outside. Hope this helps.
Keep an eye on them. Open your window to let in some air during the day, but close it again at night or after a couple of hours to train them.

as off today my sunflowers have true leaves, will re-pot them tonight. but now some others are dying of damping off. can i rescue the other seedlings from the same tray or is there no point since it's a fungal disease?

Remove any with signs of damping off. Put the rest somewhere a little less humid and with good air movement and ventilation. Outside is ideal. Let the surface of the soil dry out.

I prefer to plant sunflowers directly outside, its just too hard to grow strong ones inside.

You can always try,
How close did you plant them? You should get more air circulation if you are getting damp and hold off too much water. Try potting them up individually and giving spray of water with a few drops of tea tree and citronella.

last night spent hours looking around different web sites (again!) about damping off and leggy seedlings. all my seeds where planted individualy and the stems are not rotted or black or anything else. they are all in perfect condition EXCEPT they have very thin stem and they have nearly all falling over. that includes spinach, parsnip, africain marigold, petunia blue, coriander, corn, leek (very poor looking!). My conclusion, even if on a table that in a south facing window, they don't have enought light. Is your conclusion the same? SO, what is my next step before i loose them all? i'm bying a second plastic greenhouse, should i put them in there? the temp. is about 10-16 C at the moment and nights don't drop below zero...or just trow them out and start again. please help

When Seedlings are leggey, it means they are growing too fast.
Sounds like too much heat to me. Are you getting not just light but full sun? Too much light is also sometime not good for seedlings. I put my seedlings in a east window to get the morning sun. No sirect sunlight. Also with alos of air circulation, not green house, but also not in direct draft.
Trust me, I grow everything in first floor flat, inside and out. Damp means not enough air. Leggy means to hot.Too much water.
Pick off all the seedlings that are rotted. Also sparce out the rest, pull out enough seedlings to clear a space around an individual one. Of course choose the strongest looking one.
Have patient. You are trying to grow them too fast.