I walked the dog up the lane and found a lamb leg, completely stripped with just the foot in tact
If you mean the skin stripped off, then that sounds like badger. If just the bones left, then could be fox.
A single lamb with mum should be ok, mum can chase a fox away. It's when she has twins it's harder for her to protect two. But if you've safe accommodation, by all means keep them in.
BH always reckons a newborn needs to be latched on and sucking for 60 seconds to get enough colostrum.
To check if a lamb has had enough, hold it up on its hind legs and look at its belly. Plump and round is what you are looking for. Concave is not good.
Check its ears and inside its mouth - should be warm.
Wake it up and make it stand - it should stand and stretch. If mum won't let it suck while you are there it doesn't mean she isn't suckling it when you aren't looking.

Great to have an expert give you the

while you're learning, though.