Please could people STOP encouraging owners to worry so much about their adult pigs being outside during winter! mallethead Cannot emphasise that enough! I know the worst has come and gone for the majority of the UK but let me explain something...
I had to put my older girls and 2 of my lone males back outside in hutches about 2 weeks ago for reasons too long to explain, but they were out there when it was -16 the other night here, and they were all fine, with no covers, just extra hay and food and warm water. I brought them in again yesterday due to various reasons with their new hutches (not weather related at all), and checking their ears and paws, they were warm enough.
You should only worry if they have never lived outside before and are being put straight out without being given a good few months of warm weather to adjust properly. If they are baby/young pigs, or very old or ill, they shouldn't be outside at all either as they are more likely to have problems, but healthy adult pigs WILL be fine! Its actually the summer you should be more worried about your pigs, of any age. Long-haired pigs MUST have their fur cut back and all pigs must be kept in cool, well-aired hutches/cages.
Sorry to rant but it needed saying as it is never emphasised on here. It's also not directed at the original poster, its just a general post to all, but this kind of talk does seriously annoy me and it's so disheartening to read each year. If you do things correctly then there is nothing to worry about, trust me, had enough experience with outside pigs to know!
