I have just sown my onion seeds this week and I have followed the advice from Robinsons regarding temperatures.
“Germination should take approximately 2 weeks at a temperature of 12deg C. Keep the seeds moist but never over water at this stage.
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We have found that a higher germination temperature will in many cases damage the seeds, thus giving poor results.”
http://www.exhibition-seed.info/cultivation.htm
Robinsons are similar to Medwyns albeit not as famous. They have specialised in onions for many many years and it chimes with Charles’s point regarding temperature being too high.
I haven’t strictly followed the 12’C max as I am not growing for the show bench,- and many other texts say 18’C is fine, but I am trying to keep them coolish despite my propagators not having thermostats.
Good luck with your second batch