You can't really see much of the lightsaber prop to identify it. I assume it's a stunt lightsaber, used for practising the fencing moves, It seems to have a metal 'emitter' with a black band running through it. Doesn't look like the Vader ROTJ saber at all.
Luke's saber is still on the chair, and Peter is showing what Mark will do once he force-grabs the saber and strikes toward the Emperor, so David should be in the Hero costume standing next to the chair ready to strike.
hey! He's from Gosport, UK, just a few miles from where I live. Always thought it funny I had Alec Guinness live about 15 mins one way from where I live and Anderson was about 20 minutes the other way. Two sides of the Force in such a small area.