Yes, remove any additional protection other than the cold GH itself and pray for some cold weather to keep them back.....although cold draughts are not a good idea.
As regards the hardiness of these little blighters, many times I've had the soil in their pots being a frozen block and yes, they've even been drooped over as if they were finished...but as soon as the weather warms-up enough to thaw them out, they pick themselves up and are stronger than ever.
To reiterate, frost won't kill them bit icy winds/draughts may well harm them.
Posts
Is it to late to sew them in a cold greenhouse ?
I always think if you haven't sown your seed by December, you might just as well wait until spring.....just my opinion of course.
I think I will wait for spring , getting rather cold at moment
Many thanks for your thoughts
David K I have 5 sweet pea up today, so I am getting there with mine, I expect your are up and a nice length any pics of your so far
Gardengirl.. wrote (see)
Yours are going to be just fine, trust me.
Mine are doing okay....when I remember to look at them. 
Thanks David K
Checked today 15 little seedling popping up so exciting they are doing ok
Gg - Just to say (if you haven't already done it) any cover over them ie newspaper glass or the like, should now be removed from those seedlings.
Just a plastic lid - they are still in the plastic gh with the door open - should I remove the lid or keep it on? cold frame being built still
Yes, remove any additional protection other than the cold GH itself and pray for some cold weather to keep them back.....although cold draughts are not a good idea.
As regards the hardiness of these little blighters, many times I've had the soil in their pots being a frozen block and yes, they've even been drooped over as if they were finished...but as soon as the weather warms-up enough to thaw them out, they pick themselves up and are stronger than ever.
To reiterate, frost won't kill them bit icy winds/draughts may well harm them.