A garage or shed is fine. I normally wait till the frost browns the stalks, shake off the soil, add some fresh compost mixed with sand, dampen it slightly then put the tubers back in the pot. That allows you to make sure the soil is not infested with grubs that will eat the tubers all winter. Keep and eye on them over winter, making sure they are not too dry. Take them out again in spring.

Dahlias always successfully overwinter in the ground here in northern France, despite winter temperatures of minus 20C over several weeks. That was something we could never do in the UK in the west of England, as it is NOT cold which destroys the tubers but WET and cold together.
So as Kildermorie says you should have no problems overwintering in the pots as long as the tubers are kept in slightly damp compost. Vine weevils might well be a problem in pots, so checking the tubers before storage is probably a good idea. Don't be afraid of cold as long as it is not so frosty that the pots will be frozen through.

You can take them out of the tubs if you like and overwinter them in a frost free place either wrapped in newspaper or in a tray of sand or dry compost. Replant in fresh compost around March.

thanks very much for the swift replies sounds like i will have some very nice tubs again next yr thanks again for all replies ! David