As far as I know, Knorr have not brought out a lamb stockpot, otherwise I would probably recommend one of those, on the basis of my experience with their chicken stockpot and beef stockpot range. But in the absence thereof, I would have thought that your local butcher was your best bet; just ask him to set aside all the lamb bones, offcuts, waste, etc., that his customers don't want to pay for, and boil those up. In my experience, nothing can equal the flavour of freshly made lamb (better : hogget, sheep or mutton) stock. Or maybe goat bones, if you have a local Halal butcher.
** Phil.