Hmmm....what you're describing sounds more like some kind of lens flare, not a light leak. Light leaks usually manifest themselves as a color-band down the photo, like this one I randomly stole of the interweb:
They aren't necessarily as bad as that though, and their location on the frame can vary depending on where the light is leaking in, but they will usually be consistent from shot to shot.
Here's a thought just to rule out light leaks completely: take several shots 4-5 seconds each (use B) with the lens cap duck-taped on in bright sunlight and with the camera back/front/sides angled variously towards the sun (yeah I realise that's a challenge here in the UK in January). If they all come out truly black, you don't have a light leak and it's something else.