According to Gas safe its this is ok to fit a cooker ntcs so that people moving down market from house to flat for example are not penalized.
Also found out today that you can use LPG cooker hose on NG but not the other way round.
you need to get your terminology correct, a second hand appliance is not an existing appliance and are seen differently in the eyes of the regs. A customers existing appliance can be moved and fitted to the British Standards if the manufacturers instructions are not available, a second hand aplliance cant and must be fitted to its manufacturers instructions.