A correspondent of mine asked this question recently. The answer is to be found in Philippa Pullar's biography of Harris, where she says he was interred at the British Cemetery at Caucade, Nice - Section G, Row No 11, Grave No. 1 - "high up in the aromatic hills, with a view to the sea, shaded by an olive tree".
The burial ceremony took place three weeks after his death (as usual Pullar omits to provide the exact date, but it must have been mid September 1931) , the delay being necessitated by his widow Nellie's lack of funds which had be remedied by some unnamed friends.
Update: thanks to my correspondent Joe Joyce I now have pictures of Harris's grave and the surrounding area of the cemetery which I've added to the Image Gallery. Click on the thumbnails below to view them.