I took a good look at some of the photographs that I took yesterday and I noticed that parts of the image are a bit blurred. So I checked the lens surface and I was shocked by the amount of gunk on the glass. I don't remember wiping my nose on the lens (ugh) but It sure looked as if that was what I'd been doing. The lens was filthy and it was only after a bunch of wiping that I managed to rid of the muck.
I'm not a big fan of lens caps, it seems to me that they are a great way to transfer dust from the inside of your pocket to the surface of the lens, but I'm going to have to be a bit more careful in future. Although the pictures haven't turned out too badly.