Is 12V or 240V more efficient for a domestic solar powered lighting circuit?
My question is, should I stick with a conventional 240 V lighting circuit, or is it more efficient to go for 12V lights, bypassing the inverter? So I guess it's the payoff between voltage drop on 12V circuits vs power loss through the inverter.
The house has no mains supply, it's solely solar with a backup generator.

Given the two items in bold above my recommendation would be to go 12 V.

You can then switch off the inverter which is going to waste power in standby. If you use 240 V AC you will have to leave the inverter running in case someone switches on a light. The 12 V option allows you to switch off the inverter at night, etc.
12 V lamps are readily available, won't need local power supplies and are simple to wire and maintain.
You may require DC-rated switches if the currents become significant.
In the event of a battery failure you can jump-start your house lighting from a vehicle battery or 12 V charger from your generator. (Check to see if it has a 12 V output too.)