Both Lisa's grave and the BBC documentary "In Our Time" state that she was born in 1921 and died in February 1943, meaning that she was at most 22 at the time of her death. However, in Lisa - Codename Yvette (1977), her aunt Louise stated that she was 24.
The date of Lisa's death, which took place in The Hostage (1978), is given as February 1943. However, the events of the preceding episode Be the First Kid in Your Block to Rule the World (1977) were dated to April 1943 in Just Light the Blue Touch-Paper (1979).
When Monique is awaiting her return flight to London (which she ultimately doesn't board), it's called on the PA as operated by British Airways. The action is set in 1969 but this company wasn't founded until 1974.