Cost of replacement is where the law will fall.
Future lost income is not fair or proportionate - as if the lamb was worth £60 at weaning, buy a replacement of same bread / type at weaning, and then your back where you were and can breed from that, so asking for £500 for 6 crops is not fair, as you can still get those lambs.
Its also too early to ever hypothesize whether the lamb would of made the grade as a breeder,
Simply put - they should make good the loss, and put you back to where you would be had the lamb not died - so cost of buying a replacement live animal of the same type at weaning, if she was for breeding, or market value for a fat lamb of the type when its at slaughter weight, + the Knackermans costs.