A pensioner has been found guilty of the vicious rape and murder of a teenager nearly 50 years ago in the oldest double jeopardy case in England and Wales.Dennis McGrory was 28 when he sexually assaulted, stabbed and strangled 15-year-old Jacqui Montgomery in her home in Islington, north London, in 1975.The next year, he was tried on a circumstantial case and cleared of murder on the directions of a judge.Justice finally caught up with McGrory after swabs from Ms Montgomery’s body produced a one-in-a-billion DNA match decades later.