Whew, Gerry, they should have been lifted in October! For whatever reason, you didn't do this. I don't know where you live in the UK but most of us have deep snow on the ground. The snow is insulating the ground beneath it and the potatoes are as safe as they can be so I would now leave them and harvest asap after it clears (lift the odd root for immediate use if you wish). If the ground is clear of snow but frozen then I would get them out immediately. The potatoes do not like to be frozen and will quickly rot.
Good luck.