Dear BB,
It may also depend on how overweight you actually are - from your BMI, probably not very? My experience with low-carbing for weight loss is that it works great on the weight you really need to lose, but not on that extra half-stone or so that you would *like* to lose for vanity reasons - in other words, maybe our bodies settle naturally at a weight which is healthy but a bit above what we would like to see looking in a mirror with a modern fashion-critical eye. I zoomed down from about 10st 10 to about 9st 6lbs, (which on most tables has me on the border between OK and overweight) and I seem to potter up and down around that weight now. I'd love to get down to 8st 12, which is what I weighed in my teens, but actually I'm pretty content where I am.
Since low-carbing is relying on the body's natural systems to work, rather than on artificial calorie restriction, and since some evidence suggests that being slightly 'overweight' is actually protective in some ways, that would make sense to me. I can see it's frustrating for you to actually put on weight though!