If I wanted to retro fit rear usb's and a reversing camera, assuming I buy the Skoda parts, does anyone know if it is then simply plug and play? i.e. the harness has the required connectors already in place, or does it require much more than just the hardware?

You have to run the wires yourself, wiring for non-fitted options is not included by the factory.

You can buy the OEM camera. The loom will probably third party and will have to be routed to the front of the car and wired into the quad lock connector. I’ve recently done this and will be getting round to writing a guide once things calm down at work.