The light isn't working? I replaced it with a second hand one and this isn't working either? Fuses seem fine?

You'll need to check the brake light switch. Most have separate contacts and connections for the third brake lamp (sometimes referred to as a CHMSL or Center High Mount Stop Lamp)

https://www.askmehelpdesk.com/cars-trucks/when-asking-about-your-vehicle-303903.html

Thanks for advice Captain Rich. I have a 2006 Honda accord estate diesel. How will I know if the brake switch for the CHMSL needs replacing? Wouldn't all the brake lights fail if this switch has gone? Thank you.

You say you replaced it with a used one... you mean the bulb itself was used?

It could be a bad bulb or a bad socket.

Thanks for advice Captain Rich. I have a 2006 Honda accord estate diesel. How will I know if the brake switch for the CHMSL needs replacing? Wouldn't all the brake lights fail if this switch has gone? Thank you.

In order to diagnose the system, you'll need a readable schematic of the system and a quality DVOM.
You can actually have portions of the brake lamp switch working and portions that are not working (some lamps on the circuit will light while others don't) plus they can have separate grounding points and those need to be inspected.

Also, where is the CHMSL mounted? Often if that lamp is on the trunk lid or spoiler, every time you open and close the trunk lid, you flex the harness and that can lead to an open circuit to any lights or accessories mounted on the trunk.