very sad to hear, sorry for your loss nats, like everyone has said you did your very best and gave the poor little thing the best chance possible.
I work for the local authority, in pollution control. If you want to persue this case, i would suggest calling your local council and reporting the problem to the relevant department, all councils work slightly differently so i'm not entirely sure which one would be best. Like i said i work for pollution control so we don't deal with those kind of issues, but you could report it to trading standards. Obviously the health and welfare of these animals are very poor and trading standards could get involved as the shop isn't providing healthy animals to the customer. Definately something that they could investigate, especially if the environment isn't sanitary and causing a health risk.
Even if they feel they can't do anything about it, they will definately point you in the right direction!
I wish you all the luck in bringing some justice to those poor animals!