What is a "boned lamb leg"? I'm cooking a lamb recipe from Donna Hays' book "Fast, Fresh, Simple" and it calls for a "small boned leg lamb" (1.6kg). I saw "bone" and "lamb" and went out and bought a lamb leg roast on the bone. 

What exactly is meant by "boned"? Does this mean the bone is removed? If so, how do I go about removing the bone from the steak I have, and should I butterfly it?

Also, the recipe recommends cooking for 25 mins at 350F after browning, but the piece I bought is 1.2kg with the bone in, so likely closer to 1kg with it removed. How should I adjust cooking time?

Thanks.
If a recipe asks for a boned piece of meat, this indeed means that the bone is removed. I think the easiest and cleanest way of doing this is just asking your butcher. As for doing it yourself, there a some videos available online, e.g. this one.

Since your meat is a bit smaller, I would suggest to lower the temperature a bit (to 340F) and leave it in the oven till you have a good internal temperature. I think it would take about 20-25 minutes, but this is just an estimate.