1. Make sure you get a lamb that fits in your slow cooker! I have a standard size round slow cooker and I was able to fit in a 1.9kg/3.8lb lamb leg which had the shaft partially cut so it bends which I got from Woolworths. In Australia, you can purchase half lamb legs (also Woolworths) which come without the shaft /bone which will fit in easily into a standard slow cooker. If you have an oval slow cooker, you can probably fit in the lamb leg even without the shaft cut.
A 2kg / 4 lb bone in lamb equates to about 1.5kg/3lb boneless lamb, same cook time.
If your lamb is about 1 - 1.25kg / 2 - 2.5lb, you can reduce the cook time to 8 hours. If your lamb is larger than 2.25kg /4.5lb, increase it to 12 hours.
2. If you use chicken broth, the gravy is slightly lighter in colour. The reason I use broth and not just the lamb juices is that lamb juices are very...well, LAMBY. :) Too lamby for gravy. I find that 2 cups of broth combined with lamb juices = perfect gravy flavour.
3. OVEN: See the Slow Roasted Lamb Leg
4. Servings - 2kg/4lb bone in is about 1.5kg / 3 lb of meat which reduces to about 1.2kg/2.4lb of meat after cooking. This serves 4 generously or 5 for normal servings (in my opinion!).
5. Nutrition - This is a conservative calculation because it doesn't take into account the lamb fat and drippings that are discarded (which accounts for plenty of the calories!). Unfortunately I don't know how to estimate that, so I've not made an adjustment to exclude discarded fat.