I wouldn't cut back the leaves - its a pity you don't know which variety they are, quite a few are fully hardy in the ground (though not necessarily in pots). However, as you don't know, move the pots to somewhere frost free for the winter - but if your greenhouse is cold and the winter is another arctic one, how sure can you be that they'll be frost free in there?

Mine is a non hardy variety..growing in a pot..sorry ,don't have the label now..but I stored it in a plastic growhouse last winter..wrapped in fleece,after the leaves had died down naturally,and the plant had dried out..The previous year,it was in the shed,same treatment ..so it has survived the worst two winters we have had for a.long time..good luck..

thanks both of you, i appreciate your advice.