It does also depend on what type of merge it was. For example if it was a marked merge say where you are being guided into the other lane, you are ahead then yes the car behind should've dropped back, but at the same time you still need to be sure the driver has. If it was just a 2 lane road getting narrower then you would need a signal to pull out in front of another car especially as you were on the left coming out to the right, which is the side you can legally overtake, Its like Stevie said you need to have an independent whitness.
As you say Dino the left lane is usually to go straight and left only, and the right lane to only go right, but as the road was being re-turfed there were no road markings. The other driver can't then be wrong for going straight from the right. But its worth pointing out to the insurance company that this is the normal lane discipline.
Dino, you also say the lights turned green on your approach, the other car was stationary. What it could seem like is that you undertook the other vehicle and tried to cut back in again the other side of the junction before you ran out of road/lane? Even if the right lane is usually only to turn right if there are no road markings the other driver can't be wrong for going ahead from that lane. If the markings were present it'd be more or less his fault. Also if you're used to the road and know how the road works, did you just automatically expect him to turn right as you approached the red light turning green?