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I have a small plastic greenhouse (unheated of course) can I use this instead of a cold frame for my seedlings to overwinter in?

Basically yes but it depends upon what type of seedlings you have.

A bit more info would help me to clarify my reply.

Sorry they are  alyssum, aubretia, dianthus, candytuft, & campanula

QuoteSorry they are  alyssum, aubretia, dianthus, candytuft, & campanula

The true answer is I don't fancy your chances but then again a lot depends upon the winter we have and how exposed your garden is.

In my opinion; Alyssum saxatile might survive but annual alyssum wont I don't think.

Aubretia might providing the pots/trays they are in don't freeze.

Depends upon what Dianthus you have, maritime won't I don't think barbitus (Sweet William) might, perpetual....no chance!!.

Candytuft is quite hardy but like Aubretia depends upon whether the roots get frozen.

Campanula is like Dianthus it depends on what variety you have.

I hope I am proved wrong and all I can say is; the best of luck

May I suggest you look them up on my website and prepare yourself for more sowings next year.

I think I might give it a go it's in a very sheltered part of the garden & also I am down on the South Coast where autumn often just wanders into spring.
Thanks for your help.

I would also use some fleece directly over the seedlings.

Keep the seedlings up, off the ground if you can or get polystyrene packaging underneath the pots

Make a frame with short canes in flower pots filled with soil and tuck the fleece under.

In fact I did it last year and everything was fine.