Story doesn't make sense. Any steward watching either live or on CCTV would have seen the flask passed around and returned to owner.
Owner of flask primary target for removal from seating, search by police, and ejection and any further club or legal consequences. Others involved, OP included secondary priority for removal if resources permit.
Demand proof - ie CCTV evidence, and say you thought is was a flask of tea.
General Comment: What the **** is this country coming to when you can't have a drink and watch football?