The Antirrhinums are now all dead. The 'damping off' disease went through the second tray at first slowly and then very, very quickly.
I had taken the trays out of the heated propagator and put them into the unheated propagator to try to reduce the effects of the high humidity but this did not stop the disease spreading. I will dispose of the compost, sterilize the trays and start with a new batch of seeds.
I always find it very sad when a tray of healthy plants just falls over like this but it is a risk i take by trying to start the seeds so early in the year when the light levels are so low and the humidity levels are more difficult to control.