None of the supermarkets sell extra lean lamb mince so you have to go to a butcher and ask them to do you some. Most will be happy to oblige but be warned it is VERY expensive because they obviously have to use the leanest cuts to make it.
I use the EL beef mince for the donner meat and it tastes luvly, I prefer it to the lamb mince. I have it with a brown barm sw chips an loads of salad and pretend I'm having a takeaway! Ha