I can't give you a definite answer because too many variables involved. They are considered "tender perennials" which I know is confusing. They may come back or maybe not. The word 'mum' is a tip off (Osteosper-mum). Depends on the severity of cold exposure, moisture, etc. I've had mums that came back for 2-3 years, then disappeared after one brutal winter. The good news is, they are easy to propagate and self seed if you leave the ground undisturbed.

Hi, do you know which species you have ?, knowing which 1 you have would be a great help because there are about 70 species, ranging from fully hardy to frost tender, Derek.

I've overwintered a couple of tender ones before - I left them out,behind a sheet of glass, in pots, but not cut back, until the frosts came,then moved them indoors to a cool room, watered sparingly, then cut them back in March, and increased watering - but you have to keep them indoors right up till all frost is gone, so in the south, that's mid May, midlands and North, end of May. Bit like keeping tender fuchsias over winter.

You could try in the greenhouse, but if the temperature gets below 5 deg C or lower overnight consistently, you might still lose them, but its worth a try - don't cut them back though.