What is a "boned lamb leg"?
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.