This solved the problem - the 2 bind-key statements at the end
in my ~/.config/nixpkgs/home.nix
programs.tmux.enable = true;
programs.tmux.keyMode = "vi";
programs.tmux.sensibleOnTop = true;
programs.tmux.shortcut = "a";
programs.tmux.terminal = "screen-256color";
programs.tmux.secureSocket = false;
programs.tmux.extraConfig = ''
# Make mouse useful in copy mode
setw -g mouse on
set -g set-clipboard on
# bind-key -T copy-mode-vi MouseDragEnd1Pane send-keys -X copy-pipe-and-cancel "xclip -se c -i"
bind-key -n Home send Escape "OH"
bind-key -n End send Escape "OF"
'';