Very vigourus seedlings, I don't know what they are but would be inclined towards Canterbury Bells or some other similar Campanula.

Yes, they look like the Canterbury Bells which I have grown before.

Thank you. I've googled Canterbury Bells and am now excited to see them in bloom next year. Will they be fine to keep in the greenhouse and then plant out in spring?

if you can pop them outside for the day and bring in on a night for 2 weeks they will be ok outside from then on. or you could keep them indoors. personally I'd harden then off and plant out.