We tend to have a good go at pre-emptively solving the problem, before any issues occur.
With good management, we don't have many problems.
Apart from that, we tend to just make sure that the lambs are active, and lively enough and then put them out in good sized mobs as soon as we can get them out of the sheds (we need the space!). In a good sized group, there are a good number of eyes and ears to watch out for charlie.
As said, most of the time a fox will pick off a lone lamb, that's either got separated from the rest, or is ill.
Having said that, you DO get problem foxes, which will take lambs until a lot older than most folk think. We know this because of finding the carcasses at / down the earths, and also one year a local lad shot a big dog fox as it was trying to drag a lumper lamb off (lamb was half alive still).
However, for the most part, there is enough afterbirth etc around to make for a far easier meal for the fox than a lamb.
Still, doesn't hurt to do some fox control pre-lambing. That's a major part of our end of winter work, on the hill farms around the place.