This sounds like "damping off", a fungus that grows on the
surface of the soil and attacks the succulent stems of young
seedlings. That's why they fall over.

While there are preparations you can use to combat this (one sold
here in Canada has the name "No-Damp"), it is better to prevent
it from happening in the first place.

Suggested rules:

1. Use pasteurized soil for your seeding.

2. Be sure that flats and pots are scrupulously clean. If they
are not new, wash them well with dishwashing liquid and hot
water, and give them a final rinse in a dilute solution of
chlorine bleach.

3. Avoid overwatering.

4. Make sure the flats are well-ventilated, esp. after the
seedlings first emerge.

5. Sow very thinly so the seedlings are not at all crowded.

Rodger

1. ! ?
2. Oh, they've been sitting outside for the past year.
3. I think this has put paid to a few of them.
4. Flats?
5. They're one seedling per 1" square pot at the moment.

Thanks for your ideas

David