Flat Hill was a granite quarry, that subsequently became part of the nearby development at Jordan Valley and Choi Wan. Not sure why a hill would be named flat, but there you go. The cache is located just off the main path that links Jordan Valley Park with the housing development. It is outside of the park so accessible in all hours, but if you come through the park, note that the opening hours are 5am to 11pm.
There is a small car park at the park, or you can try buses 27 29M from Choi Hung, or 23 from Kwun Tong