By Dan Warburton The grieving mum oflaughing gas victim Ellen Mercer last night urged police to investigate her daughter’s death. Sharon Cook, 54, wants a probe into suppliers following an inquest which heard how business student Ellen died at just 24 after becoming addicted to the party drug. In a victory for the Mirror, Instagram has banned sellers who we revealed were offering to deliver supersize canisters of laughing gas in just 20 minutes. Now Sharon says detectives should find whoever sold the gas to her girl while she was ill. The distraught mother said: “They were delivering industria…