Originally Posted by echinosum

It won't have frozen to death in E Yorkshire.

That leaves 4 possibilities:
drought
drowning
starved
alive but defoliated

Drought: Sounds odd in winter, but in a pot, if it gets frozen over an extended period, they can die of drought. But given the weather we have had this winter that sounds unlikely, even with the recent frosts - only a few days.

Drown: if the bottom of the pot is blocked (or it isn't drained) and it got full up of water for a period, it would drown.

Starved: They do need management when grown in pots, as they are hungry plants. If you bought it from a garden centre, it may need putting into a larger pot as soon as you get it, as it may be sold in too small a pot to carry on growing. In a correct sized pot, what happens is that over a few years, as few as 2 years if you feed it well, the roots fill the pot and take up all the nutrients. They then have to be either repotted into a (yet) larger pot, or else divided (typically into 2, may require saw or axe to achive) and put back into separate pots with fresh material. A good time to do this would be spring, eg April/May, not now. Autumn also possible.

Alive: It may just have decided to defoliate given the recent wind and cold. If the roots look good, just cross your fingers it will push out new leaves in the spring.