No matter how many times I say it, I can't seem to get the message through.
Sweet peas are extremely hardy and will not be affected by frosts....of course, if they have been made tender by over-coddling, then they may need hardening-off prior to being planted out.
Louise, don't put the tops on your boxes unless you have already been doing this recently.
Not often I agree with Monty Don, but he was spot on when sowing his SP seed the other week i.e. sow the seed in pots, bung em in the cold-frame, then leave em there until planting-out time.....job done!