I am with JayG on sowing much later - 1st or 2nd week in May for me in Bradford.
I had all sorts of experiments with sowing in loo rolls and transplanting, covering with a scaffolding plank etc until I delayed direct sowing until May. Last year didn't go so well,not sure why, but before that had 5 years of great success on the trot with direct sowing in May under enviromesh to repel the carrot fly.