How much does a U.S. dollar go for in the UK? I just bought a L114 redtail sternella for $17.99.

You're supposed to quarantine any wild fish! If you dont have a quarantine tank have the lfs quarantine it for you. Make sure it's eating before you take it home. In your situation I would try to feed it-- mussels, carnivore sinking pellets, frozen foods (bloodworms, brine shrimp, whiteworms) and ...