It sounds like you're doing the right things. Grey mould loves cool, damp conditions. Ventilation is really important. On warm days, try to open all the vents in your greenhouse and the doors to get as much fresh air in as possible. Spacing is also crucial. It's always tempting to pack in as many plants as possible, but closely spaced plants have humid pockets of air in between them - which fungi love. After the danger of frost has passed you can take some of your plants out of the greenhouse altogether for the summer.
I hope you have a good crop this summer,