What you describe may well be a cat kill. They remove the innards to a short distance away, and feed from the saddle area and flay the skin from the quarters as they feed. If you skin out the neck, you will probably find some evidence of wounds and bruising.
What size of a lamb are we talking about?
It's true that leopards lift kills up into trees, but that seems only to occur where there are other predators which are likely to steal the kill...ie, hyenas and lions. In the UK, where these cats are top of the food chain, and under no threat from larger animals, they seem not to do this so reliably.
Searching the area around the kill, within a few hundred yards, may reveal some further eviidence, such as scats/prints in soft ground/marks on a strainer post where the animal has leapt out of the field/hair left on fence wires, and the like.
There are several websites and forums dedicated to big cats and evidence of their habits. For example
http://www.bigcatsinbritain.org, and this one
http://bigcatsgb.co.uk/6.html which has images of typical kills. If you look, you may find similarities.