'The First Vietnam War' by Peter M. Dunn, p87-88
https://books.google.co.uk/books?id=...erator&f=false
and
'Intelligence and the War against Japan: Britain, America and the Politics of Secret Service' by Richard J. Aldrich, p210-211
https://books.google.co.uk/books?id=...20duty&f=false
Both make reference to the alleged incident. It occurred during a breakdown in relations, one of the consequences of which was that we weren't informing the Americans of special duty flights into Indochina. Three aircraft were lost on one particular mission. Americans told the British that it was possible two of them had been shot down by P-61 Black Widow night fighters (at that stage of the war, they were shooting down more allied aircraft than enemy apparently) having ventured into range of the US air defences.
Both sides brushed the matter under carpet as relations were repaired shortly afterwards. Both sources point out that special duty missions continued so it seems more likely to be just another tragic fog of war incident rather than a conspiracy to stop the French reclaiming Indochina.