Thanks all. There has been a little confusion. I have a perfectly good exposure meter in my LX, and a handheld one. What I was querying was
what to meter. Just taking the internal light meter at face value will result in the surrounds being too light and the lights being washed out. As Phil says, I'm guessing I should knock one or two stops off the exposure (then possibly add them back on again for reciprocity failure if the times are long!). I'm beginning to veer towards setting-up the camera on the tripod and letting the LX do its long-exposure thing...
Many thanks everyone. K.