Cooking time is calculated based on the weight of the lamb cut, as well as whether you want the meat to be medium or well done.
Plan to allow the joint to sit, covered, for 15-20 minutes before carving - it allows the juices to retreat back into the meat, giving a much moister cut
I love Cottage Pie and Shephards pie. But a slow-cooked version of them leads to much, much softer mince and makes the house smell awesome all day
Out of convenience, this recipe uses a couple of Shepherds Pie spice sachets - you can make the mix up using herbs and flour if you'd prefer. One thing to note though, is that you should under-provision the water - unlike cooking in the oven, the sauce won't reduce down in a slow cooker, so it needs to be about the right consistency from the outset.
Unlike when you cook it in the oven, the top of the potato doesn't crisp up (you can put it under the grill for a bit if you'll miss that), but the edge of the potato layer does.
If you're feeling adventurous you can also add mixed beans to this, or outright swap the lamb for Beef and make a Cottage pie instead.
This lamb should ends up succulent and delicate, falling apart easily.