Sorry for the slow response here, due to technical problems on this forum. This is what I wrote to Joe by email:
The simple answer is that is makes no difference whether the sign is temporary or permanent. A diagram 670 terminal speed limit sign needs to be lit if it is on an A road (a principal or trunk road) in a street-lit area.
Most road works temporary speed limits are on rural (so unlit) roads, so they don’t need lighting. If it’s a street-lit area though, you need gas bottles, battery powered lighting, or something wired in to a lamp column for just the terminal 670 at the start of the limit.