That reminds me that sticking heads through the fence seems to be hereditary
It can't be that they are copying their mums as once they reach about a year horned sheep cannot get their heads through any more. It must be an adventure gene to explain why some ewes have lambs every year which stick their heads through the fence, while others never produce lambs which do that
I'm trying to think of an alternative to adding wire mesh along the bottom of the fence. A piece of windbreak mesh is NOT a good idea as they seem to get stuck in that even more, but if it's a big problem then sections of boarding might work - an expensive option though if you haven't identified escape as a problem yet. I know most people won't agree, but I think that in general sheep aren't as stupid as is made out and do in fact do their best to stay alive
We find that escapes tend to be made between the gate at its hinge end and the straining post, and a spare stob tied in there vertically, coming right down to the ground, can keep them in. For under gates, either make sure the ground is built up to the level of the bottom of the gate (and doesn't get worn down by traffic over time), or hang a couple of stobs by wire to the bottom edge of the gate. Check all along your new fence for natural dips in the ground where lambs can squeeze under.