You're welcome Vallee.
I also had a family member who resided in Colney Hatch Asylum - he was an epileptic (my uncle). When he died in 1939 my grandparents reclaimed his body and he is buried with his parents and sister in a private family grave in a municipal cemetery (with headstone) - so that story has a slightly happier ending.
Hope you track down where your relative was buried.
Cheers, FHA