Okay - just went back to my garden notes, and they say:
late Feb 2008 -> Seeds in Propagator (unheated, but with lid)
30 March 08 -> first green leaves coming through
27 April 08 -> ready to pot on, but Echinacea purpurea magnus quite small
So your tip about temperature could be right. The Rudbeckias took around 60 days to germinate (didn't generate leaves until late April), but once they came through they were strong & did well - so it's the earlier Echinaceas that bit the dust.
We did have snow over Easter (late March) and April was usual a mixed bag. So when the Echinacea would have come through, it would have snowed outside.
Didn't realise I sowed them that early until I went back to my notes - well, in my "gardener's memory" I just recall really nice spring days but my garden diary begs to differ! Weird thing is that we had a really mild Feb / early March - and then the snow came.
I'm originally from Austria and we have the "3 Ice Saints" - they are saint days in the catholic calender, and only once they have passed is the danger of frost over, no matter how much the mild weather might lull you into a false sense of security. Must pay attention to them in 2009!