Toot, As far as i know, the frost wont affect ungerminated seed, they just might not be warm enough to germinate. It may take a little longer.
A bit of a warm spell, and they should germinate. Its when they are up and out of the ground, and then the frost comes, that they are in danger, as the frost can just frazzle the new leafy growth. So its not too late to cover them up now, and it should help the soil warm up and help them get started.
It it stays wet and cold for too long, the seed may just rot before it gets a chance to germinate, and then the best thing is to do is to start again.
Many veg growers have held off from sowing anything so far this year because of the cold temps although itching to get things in the ground, so it's still not too late to start. Some have gone ahead and got good results despite the cold weather.
You could wait a week or so and see what happens and then sow in pots or seed trays and when it is warm, and your direct sowings havn't appeared you'll still have small plants to plant out.
Good luck, I coooked my purple sprouting brocolli tonight for dinner and it was fantastic, picked it 5mins before it went in the steamer....its so worth the time you put in when your harvesting and eating.