The Duke of Cambridge has said it was "very difficult" not to have his late mother at his wedding. Interviewed by US TV network ABC, Prince William agreed he had "missed" Princess Diana at Westminster Abbey. "It's the one time since she's died, where I've… thought to myself it would be fantastic if she was here," he said. Meanwhile, Buckingham Palace has announced the prince has been made a Royal Knight of the Order of the Thistle, Scotland's highest honour. William and his brother Prince Harry were both interviewed by ABC's Katie Couric for a documentary to coincide with the Queen's Diamond Jubilee. William was 15 and Harry 12 when their mother died in a car crash in Paris in August 1997. Asked about his April 2011 wedding, Prince William said he thought it was "just how sad really for her, more than anything, not being able to see it... I think she would have loved the day". He added: "I sort of prepared myself beforehand so that... I was sort of mentally prepared so I didn't want any wobbly lips or anything going on," he said. He said he hoped Diana would have been "very proud" of both Harry and himself on the day he married Kate Middleton. He added: "I'm just very sad that she's never going to get a chance to meet Kate." Interviewed separately for the Jubilee Queen documentary, Prince Harry agreed it was "hard" that Diana was not at the wedding. "I think she had the best seat in the house probably," he said. "She would have loved to be there." 'Very honoured' On the subject of the Queen, Harry said his grandmother was someone who is "really very very normal, very relaxed. "She obviously takes a huge interest in what we all do... she wants to know which charities we are supporting, how life is going in our jobs as such." A St James's Palace spokesman said the Duke of Cambridge, who has the Scottish title the Earl of Strathearn and is already a Knight of the Garter, was "very honoured" to be made a Royal Knight of the Thistle. The accolade is second only in precedence in the UK to the Order of the Garter and is given after a personal decision by the Queen. The spokesman added: "It's his 30th birthday this year and also the Diamond Jubilee and the Queen felt it was an appropriate time to bestow the honour." William will be formally installed as a Royal Knight of the Thistle in July during the Queen's official week-long residence at the Palace of Holyroodhouse in Edinburgh. The Duke of Edinburgh, Prince of Wales and Princess Royal have all received the same honour in the past. In London, the Duke of Cambridge's wife, Kate, attended her first Buckingham Palace garden party alongside the Queen, the Prince of Wales and the Duchess of Cornwall. She wore a soft pink dress by designer Emilia Wickstead, with matching Jane Corbett hat. It was the second garden party to be held at the palace so far this year and was attended by 8,000 guests.