....I want to power the sockets via the inverter when off grid, can I do a switchable supply or is a separate circuit best.
Really depends on how you like to think. No one way is best.
You could get separate sockets for EHU and Inverter. Simplest way. Also ensures if the mains goes out you don't run something that would be too high a power off the inverter (kettle, heater, etc).
You could get an automatic switchover so the socket is automatically on hookup when available. Convenient way to have dual use from a single outlet
Or manual switchover. Dual use with ultimate control.
Whichever way you go, it is always useful to have a couple of sockets dedicated to EHU only for stuff you would only ever use on mains hookup (THe Battery Charger is an prime example)
I have completed my mains wiring for ehu providing a pair of lights two double sockets with usb and supply to my battery charger all through a consumer unit.
Lights and USB sockets probably best supplied from DC direct for best efficiency though?