Tents are supposed to keep the weather out and the heat in. The key bit being the heat staying in or you might as well not have it. What they are made from hasn't changed much in recent years but the way they are made has, incorporating thread patterns on a minute scale to give the fabrics properties that wouldn't otherwise be possible. Cotton is known for being warm and is so because it catches air in tiny pockets, but that alone wouldn't stop enough heat escaping that it couldn't be visible on say a thermal imaging camera like the one used by the Telegraph(and now the Daily Mail) at the occupation outside St Paul's. Rather than using the properties of cotton to catch and store warm air in little pockets, it can be threaded into a form where it simply reflects that heat so in the case of a tent, the whole thing is a big warm air pocket. It's very efficient and some forms of it can actually reflect more than 99% of the heat back whilst still being a breathable material.
The Telegraph and the tabloids neglect to mention this, nor do they say if they actually went and checked some of the tents that showed up as 'cool'. A well-insulated house with furnaces burning can look empty on a thermal image where it is obvious to the naked eye that it is not. It's not really an interest of mine what the occupation does and what critics say, but if they're really having to scrape the barrel like this and rely on the basic scientific illiteracy of Britain(who also haven't watched Predator because they're hooked on the rubbish that passes for 'action' these days), then they are somehow managing to lose an argument they claim isn't even being had.
There is no way to tell from the images taken how many tents are occupied. It is the occupiers word against those that say thermal vision 'proves' the tents are empty with absolute certainty.
EDIT: when I creep around sleeping people with my thermal vision goggles, I get arrested.