if the dahlia is in free draining soil the chances are it could be left in the ground, but as we never know what type of winter we are too get i would be tempted to lift the tuber after the frost has killed the top growth, remove all surplus soil from the tuber and turn it upside down for a few days to remove trapped and stored water from within, and wrap it in newspaper in a frost free place.

If you were in the southeast, I'd say leave them in, but in Lancs you'd do better lifting them and doing as Stevebuk says.
Even if you cover them with straw to keep them protected from frost, it's the wet and cold conditions in winter which causes the tuber to rot. Here we get minus 20C yet the dahlia tubers survive as it's very dry.

Better to be safe than sorry and to have them up.

if you do take them up when they are dry give them a dusting of green sulphur and keep checking through winter to make sure none has gone bad.

hi many thanks for replies . i know exactly what to do now will store the tubers in a box in my wheelchair trike shed they should be happy and snug in there ... when in spring should i plant them ? as we often get frost quite late into spring here in the frozen north ! ... thanks alison .

Dahlias are late flowers which don't really come into their own until well into July or August, or later. The tubers won't start into growth in cold soil so planting them early is no point. What would be better with large tubers is to start them into growth in shallow trays over some heat in spring and then do dahlia cuttings from the new shoots that form. That way you can make a lot more plants which will flower well the same year.
Alternatively, just plant the tubers once the soil has really warmed up, in May or June.

hi many thanks thats great will do that ... alison