They will come through a cold winter better in the ground (roots don't get frozen solid) but in a normal winter, should be ok.

Totally agree about them being much safer in the ground - pots are asking to be frozen, obviously the roots being frozen too.
I have a lot here and they're fine through the winters - even very cold ones like the last one.

As you're in Lancashire, I'd be inclined to offer it a bit of protection, especially if we're at risk of another cold winter (which is what the word is currently). Keeping it in a paved or gravelled area is better, because it'll be warmer, which is where you have it now, but I'd be inclined to put it very close to a house wall, in a south facing position, and either cluster other pots around it for added protection, or bubblewrap the pot and get some horticultural fleece to place over it if the weather gets really cold. It's not snow you need to keep off it, its freezing that might be an issue. I had 3 on my balcony - 2 were fine, but one kicked the bucket at the end of last winter, and my balcony is southfacing and in West London - they're in pots. I'd had all 3 for over ten years with no problems at all, so in a bad year, they may be susceptible. There were people up north on here who lost Phormiums in the ground last winter, so I'd be taking a few precautions, as mentioned already.

thanks for all the advice. Will nip to the GC tomorrow and get some more fleece - better to be safe than sorry.