The theory is that Mannock's remains are in in the grave marked 'A British Airman of the Great War' 'Known unto God' at row F, grave 12 in the Laventie cemetery. This is not a certainty, but it is a pretty fair possibility.
Mannock is not short of other memorials however, so he is certainly not forgotten: Aside from the Canterbury plaque, his friend Jim Eyles placed a plaque on the house at 183 Mill Road in Wellingborough where he lodged with the Eyles family. The RAF named one of its 10 Squadron VC-10s after
Edward Mannock (now no longer around unfortunately), and there was also the Wellingborough Air Training Corps Squadron, which was named after Mannock when it was formed.
As far as more well known legacies go, most fighter pilots are familiar with not only the Dicta Boelcke, but also Mannock's own similar 15 point guide for fighter pilots, which served many of them well in WW2 and beyond. Notably, both Douglas Bader and Johnny Johnson are on record as saying Mannock was the greatest air ace of all time, which I totally agree with.
Aside from those, my Land Rover is called Edward, in my very own humble tribute to Edward Mannock, and as long as I'm around I'll continue to bore people about him, whether they like it or not LOL, since he was a man whom I admire very much indeed.
Al