I always sow my sweetcorn in first week of April in the greenhouse. I have grown for many years now Golden Bantam.
Find they germinate after a week and by early May, well now are about eight inches or so high and ready to plant out. Will be planting out mine next week.
Have saved my own seed last few years and happy once again this year that all of them germinated and produced lovely strong plants.
I also sow some more in the greenhouse now and plant them out in June in a raised bed and where some of my Broad beans have been. They soon catch up with the early ones and you are picking a couple of weeks later than the earlier sown ones.
Always found germination of sweetcorn to be quick and reliable and being strong plants they just adapt easily to the move outside.
I have always grown them in long six inch paper tubes, water them well before transplanting, pulling off the wet paper. I also pull a little soil around their bases on planting out and harden it down carefully. Although they do grow top heavy and never fall over, I'm sure that helps a lot.
All they need is sun, regular watering and a good knock as you walk by each day to produce a great summer crop that looks great, tastes great and I am always proud of.