Well after test running one empty flat with a heavy rake of mixed stock behind I modified the flat to run better by the following;
1.Removed the brass flat washer between the bogies and body and inserted M3 steel washers.
2.Removed the couplings from the bogies and fitted kadees to the actual body.
I have been tested this arrangement for over 1 hour running forwards at a scale speed of 60MPH and backwards at 15MPH over various points and track configurations and so far no derailments. Please note my sharpest curve is 22"