Mr Grosvenor became the seventh Duke of Westminster when his father Gerald suddenly died nine months ago.He inherited the bulk of his father’s estimated £8.3 billion fortune, and became the third richest person in the UK and the 68th wealthiest in the world. The 7th Duke of Westminster, Hugh Grosvenor, arriving for a memorial service to celebrate the life of his fatherCredit:Peter Byrne/PA Wire Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. They apparently stayed at the exclusive Ventana resort on the Big Sur coast, renowned for its views over the Pacific Ocean.The source added: “He is happiest when he is enjoying time with Harriet and their close circle.”He was left crushed when his father died but Harriet has been a huge support and is helping him to get used to his new responsibilities.”They’re a lovely couple and deserve all the happiness in the world.”Hugh’s father died at the age of 64 on his Abbeystead estate in Lancashire. “They have a lot of shared interests and are both very down to earth and family orientated.”She’s a really lovely girl and very much the love of his life.”The pair, both 26, met while pupils at the private school, Ellesmere College in Shropshire.Miss Tomlinson grew up in Chester near the Duke’s 11,000-acre family estate, Eaton Hall. She went on to study teaching at the University of Wales but chose not to pursue it as a career. Gerald Grosvenor, 6th Duke of WestminsterCredit:Paul Grover The Duke of Westminster, Hugh Grosvenor, on holiday in California Credit: As Britain’s youngest billionaire, and perhaps the world’s most eligible bachelor, the Duke of Westminster has been in high demand.But now Hugh Grosvenor is officially off the market after being pictured arm in arm with new girlfriend Harriet Tomlinson.The pair were seen smiling on a romantic getaway abroad. At the heart of the family fortune is a property portfolio that includes 300 acres in two of London’s most expensive neighbourhoods, Mayfair and Belgravia.As well as property and land holdings in the capital, the new Duke inherited huge tracts of land across the British Isles, including in Oxford, Cheshire and Scotland, as well as in Spain.A source close to the new Duke – reportedly the richest man in the world under 30 – told The Mirror: “Hugh and Harriet are blissfully happy. Instead she became a consultant on the estate agency desk at recruitment firm Deverell Smith in the City of London.Meanwhile, the Duke works as an accounts manager at a green energy firm.Miss Tomlinson and the Duke have apparently taken a string of romantic trips abroad including to California where he is pictured posing in front of a bright yellow Ford Mustang.