Princess Diana agreed to her explosive TV interview with Martin Bashir because she wanted to teach her sons the importance of communicating on a deeper level, a never-before-seen letter claims.The princess opened up about Prince Charles's close relationship with Camilla Parker Bowles, stating memorably there were 'three of us in this marriage'.After the 1995 interview, watched by 23 million people, one viewer, Michael Barratt, wrote her a letter of sympathy.A week later he was astonished to receive a hand-written reply saying how touched she was and telling him that she hoped the BBC Panorama programme would help women in similar difficulties.She went on to say that she wanted Princes William and Harry to learn from it too.The two-page letter, written on November 27, 1995, on Kensington Palace-headed paper, is signed, 'With my best wishes, Yours sincerely, Diana'.It has stayed in Mr Barratt's possession for three decades and is estimated to sell for £3,000 to £4,000 at auctioneers Reeman Dansie of Colchester, Essex.Auctioneer James Grinter said: 'Michael Barratt watched the interview on November 20, 1995 and was so moved he sent Princess Diana a supportive letter telling her how hard it can be to move on and that life can be better in time. A letter has claimed Princess Diana agreed to her explosive TV interview with Martin Bashir because she wanted to teach her sons the importance of communicating on a deeper levelThe two-page letter, written on November 27, 1995, 'To his great surprise, a week later, he received this wonderful reply. This has got everything you could possibly want.'In the same sale is a rare Madame Tussauds sittings sheet for Diana dated April 18, 1996, in which the museum's team assessed the princess at Kensington Palace before creating her waxwork model.Observations include how her hair was in 'very good condition' and the second page reveals she used Lancome black eyeliner and Estee Lauder rose lipstick.Mr Grinter said: 'This is a unique opportunity to study the styling used by one of the most famous women in the world.'The sale will take place on June 9, with the sittings sheet estimated at £500 to £1,000. Diana's interview with the now-disgraced journalist Bashir hit headlines again more than two decades after its screening following revelations that Bashir had used forgery and deception to talk to the princess.A 2021 inquiry into the intimate conversation by retired judge Lord Dyson, titled the Dyson Inquiry, revealed that Bashir had spun a great web of deceit in order to gain direct access to her. Bashir showed Earl Spencer, her brother, false bank statements implying his former head of security had been receiving money from tabloids and the security services to spy on his sister.Diana's interview with Bashir was said to have had a detrimental impact on the Princess' relationship with Prince William Once he had gained access, Bashir told Diana a series of lies, convincing her that Prince Charles was having an affair with then-royal nanny Tiggy Legge-Bourke - now Alexandra Pettifer - and that she became pregnant and had an abortion.But even before such lies were exposed to the world, the interview was said to have had a detrimental impact on the Princess' relationship with her beloved son, Prince William - despite Diana's best intentions.Writing in Battle of Brothers, Robert Lacey described how when William, then aged 13, watched the interview alone in his housemaster, Dr Andrew Bailey's office at Eton, he was later found 'slumped on the sofa' with his 'eyes red with tears.'In December 1995, the Queen urged Charles and Diana to divorce and in August 1996, they finally did.Less than one year later, Diana and her lover Dodi Fayed died tragically in a car crash in a Paris underpass.