I leave mine until they get the first frost nips. the foliage blackens. I leave the pots in a cool greenhouse on their sides to dry out. I then just leave them until March when I knock them out and repot and start watering. Some years I have put the tubers upside down to dry and then pack them in sand. both have worked for me.

It is not cold but the wet which destroys Dahlia tubers in winter. We never dig ours up even though we have winter temperatures of minus 20 C here; the difference is we don't have the persistent rainfall and damp. I am amazed sometimes how they return after a cold winter.
So for storage, it is sufficient just to leave them in their pots protected from the worst of the frosts. An unheated greenhouse can become just as cold as the outside if not heated at all, so you are better to store them in a garage or shed where the temperatures remain at or above freezing.

Thank you so much Seaburngirl and Bertiefox for the advice . I feel much more confident now x