If they're "nice big plants" they're probably variarions on C. persicum - which is less hardy than C. hederifolium and C. coum. I'd just take the risk and plant them out if that's what you were going to do. Sometimes they come through and sometimes you lose them, go for it!

P.S. There are more colour variations with C. persicum - makes them very popular as winter bedding.

It's usually wet and cold that kills them, rather than just cold. Pull off the dead flowers at their base - otherwise they rot and cause the other flowers and leaves to rot, too.

the large leaf/ flowered ones are indoor plants and will not survive the colder temperatures. by large I ean more than 3" across for the leaf size. any you buy die because they have been in the draughts near the doors of the supermarkets.

Chris Grey-Wilson gives an excelllent tongue-in-cheek test. Plant them outside; if they survive then they are hardy, if they die then they weren't! ( He was actually talking about the identity of C. hederifolium and C. africanum)
More seriously, I tend to agree with the above and think that your 'nice big plants' could be C. persicum cultivars and are house plants.