When you water the trays check out the bright orange water than comes out the bottom - that cannot be good for your little ones?? My dear old Percy had survived my Mother's dubious "care" where he was chucked outside in his pot for ten years and had the contents iof the teapot emptied on him daily. I took him when I left home, he'd been fine for decades until I mistaken put him in that bloody stuff thinking it was good!!! He's back in ordinary compost now and thankfully his one leaf survived the ordeal and is sprouting again
I chucked the bloody lot of that horrible stuff into a flower bed and its still "glowing" in there!
Well, almost all my seedlings have come up already after just a few days I'm hoping @TheDragonWithAFlagon is correct with your Croydon analogy, and also that they do OK when I move them to the Home Counties (aka some nice John Innes no. 2)