So I'm changing my rear brakes this weekend, and in a cloudy haze, I went and did some thing stupid and hlafway unscrewed the bolt holding the e-brake cable in, and I felt some sort of pressure release up. I tightened the bolt back up and replaced the brakes but now the e-brake has lost all control of the wheel, it seems, and is very loose where you engage it.

How do I go about re-tightening it all up?

I'd guess you will have to remove the disc/drum to have a look at what has jumped off what....I reckon that's the only way to do it properly. I'd do it soon before you damage something inside the drum.

The end of the cable may have slipped off the lever it pulls.