Any question answered...

does anyone know if there is bike parking on any platform at Tottenham Hale Rail Station?

According to TFL the is bike parking, but it doesn't specify if that's in or outside the station. I want to leave my non-descript bike there for four days with some amazing locks, anyone know if that would be ludicrous?

Just emailled you a screen grab of the bike parking at TH. Its right in front of the main doors to the station next to the taxi rank by the looks of it. As far as bike parking goes, it could be a lot worse... Maybe take your saddle and seatpost with you if you are really worried?