i am in se london ish but classd as sevenoaks. the shop is called birtchwood aquatics. it looked like a good shop the people where helpfull and the tanks in there where all clean and the shop was always clean. every othere shop i went to the tanks and shops where filthy and didnt feel it was good to buy any fish from there in case they had any dieases etc ...
If that is the shop that has the best looking livestock then i would continue to use them. As a general rule i tell people never to just take the advice of any fish store employee because it is all to common to come across stories like this where people have been given bad advice. The lesson here is do all the research yourself or ask here for help before making purchases. I made similar mistakes when i first started. I would still suggest taking back or selling all the fish besides the clowns. We can help you find other fish that are more suited to your tank.
Very sorry about the loss of the tang. It is always difficult to lose fish.