i never dug up my dahlia bishop of liandaff will it grow again

Keep an eye on it. They can be left in the garden all year. I did this last year with know problems. Its the wet they dont like not the cold.
I lifted all of mine this winter but I have noticed a shoot growing outside where the tuber must have split when I lifted them.

Mine have been slow this year. Fingers crossed for yours.

I scraped the soil back where my normal dahlias are left in and they are alive. but they were burried very deep. My brother in sunderland dug his tubers up by mistake last week and he said they were just mush. But they werent very deep.

so you may be ok give it to mid/late may before buying replacements.