Thanks to you both that is super helpful!
As it happens, the current concrete level is a fair bit above the grass level (about a 10cm step down), so I think I may try dig around 5-10cm for the area I am doing gravel/patio, and use this to build up the area I am not and try to end at the same level.
Great tip re the Geotextile too, I was planning doing a weed membrane between the hardcore and gravel/concrete, but hadn't considered under it.