An incredibly thorough and helpful insight, thanks guys!
I'm fairly certain I'm lucky and have the enhanced option (after having had the reg checked for a full list of options).
Re the CD changer, didn't realise the wiring was fitted regardless and that explains why it has the wiring but no changer. I also thought it was placed rather peculiarly.
Simply put, I would like to try a new Kenwood unit under the drivers seat (I very rarely have rear seat passengers and the unit is still fairly small), which already has a built in amp. How would I go about connecting it to the existing audio setup, is there a way to tap in to the speakers without causing too much aggro?
https://www.amazon.co.uk/Kenwood-KSC.../dp/B00BO8O9XQ