LONDON   -  Queen Elizabeth II returned to royal duties at her Windsor Castle residence on Tuesday, carrying out her first engagements since fears emerged that she could have contracted coronavirus last week. The scare arose after Prince Charles, her eldest son and heir to the throne, tested positive for Covid-19 for the second time, two days after meeting the 95-year-old monarch in Windsor, west of London. His wife Camilla, the Duchess of Cornwall, also then tested positive, and the couple have been self-isolating since, according to aides. Buckingham Palace has not revealed whether the queen has taken any Covid test herself. On Tuesday the monarch -- who this month marked 70 years on the throne -- held two virtual audiences from Windsor, meeting via video-link with newly appointed ambassadors from Estonia and Spain.