Tips for cooking a whole lamb in a fire pit?
In the past when I've cooked in the ground I put rocks into the fire. Don't really know what sort of rocks, but I've been involved in sessions that use bricks. 

Point is that you needs some way to "envelope" the heat around the thing you are cooking so what we did was put the rocks into the fire to heat them up.

Carefully remove the rocks before putting the lamb in and then put the hot rocks on top of the hessian covered meat before putting the dirt back on top.

Let it sit for a while and you should get a much more even result