If the seed packet says sow in March, then put them in. NOT all of them though. Then you have some in reserve, carefully keep the rest, just in case.
The days are warm but the nights can still get chilly.
The slugs are about too and they adore a buffet of new seedlings.
You can cover your babies with a bit of glass, fleece, clear plastic, but make sure there is a gap so that when they shoot up the leaves do not touch the plastic covering or that will kill them too.
I suggest a few seeds in a flowerpot on the window sill. Not too many or they will be impossible to separate. Twenty would do for most. Then as soon as they get to a couple of inches high, gently turn the flower pot on its side and slide it off, you can then take out each seedling and give it a pot of it's own. You could then stand the pots outside somewhere safe and let them grow on.
I got a clear plastic yoghurt container from one of the supermarkets, it had 12 dividers in it and I put about 20 seeds of all different brassicas and things in separate flower pots which fitted the container. To show a new plot holder how to start, they took it home to put on their window sill a couple of weeks ago. They brought it back this week end to show me and many are coming through fine. Next week end I should think a few will be ready for stage two.
The set was:-
sprouts, Evesham special
round cabbage, Primo II
pointed cabbage Greyhound
leeks
Purple sprouting broccoli
Cauliflower All Year Round
Lettuce All Year Round
Marigolds french dwarf
Allysum
6 sweet peas (mixed)
1 marrow (A bit soon and it is not through)
some pepper seeds from a pepper (again not through)
He was chuffed and so was I.