The only place I have found extra lean lamb mince is Waitrose but it was way too expensive. I always use the extra lean beef but it is nowhere near as nice as lamb. I have used reduced fat lamb mince and synned it before and it is tastier and much less dry than the beef.
When I do the beef mince version I find it only needs 30-40 mins in oven and I make plenty of garlic sauce or homemade coleslaw which works really well with it