By Susie Beever Love letters penned by Princess Diana to lover James Hewitt could reportedly be sold for up to £780,000 in what has been dubbed a “final insult”. The 64 letters were said to have been written to the ex-military officer between 1989 and 1991. Mr Hewitt claimed he had previously offered the letters as “collateral” for a £500,000 loan. The decorated former cavalry officer had an affair over several years, while the late-Princess was still married to King Charles. It’s understood the pair shared letters while he served in the Gulf War. The letters, which disclosed intimate details …