Yesterday we put one of our custom files on this commercially used 02 plate Discovery TD5. The customer pulls a 3.5 ton trailer behind it and wanted more torque to do so safely. We read and write the custom file through the OBD socket on these and it doesn't take long at all.
It turns out this one had been remapped before much to the surprise of the driver but once we finished writing our new improved file to the ECU he couldn't believe the difference. End result - the happy customer now wants his personal car remapped, job well done!