Robgo, in your situation that you describe, it is too late to move over and let him by. You would be converging together at the apex, thus getting in the leader's way. You would be forcing the lead car to take a different line than the race line at the corner. He would have to take the corner wide, when it should be you taking the corner wide to let the other car through.
The only time you should be moving offline like that would be with plenty of distance left between you and the lead car so he could see your intentions and plenty back towards the beginning of the straight. If the lead car is only 6 meters behind, no matter where on the track, then that is the time the lead is planning and getting ready to execute how he is going to pass you. If it was me 6 meters behind you in that situation then I would have to take evasive manuvers not to crash us as you would definitely be diving into my prepared and planned path.
In normal blue flag race circumstances, just hold your line. You will see when the lead car attempts to make the pass and when he does, lift off the throttle to make it easy. Then both of you continue on your way with minimal impact to the race or to lap times.
In midjoin blue flag circumstances, the best is not to let that happen in the first place. Sit in the garage and wait for a break on the map to allow you to lap by yourself. If someone catches you on the track, you have plenty of warning to pull off line and SLOW DOWN to allow those in the race to pass through. I'm not going to say you have no business being out on the track with other racers. I have no problem with someone joining in and getting some practice/warmup laps. But you have to have no involvement in the race, including being on the race line or in a corner when racers are on your bumper. If you have to slow down way off line on a straight when you first see blue flag, but don't see the car yet in your mirror, then so be it. That is what you have to do because you are not a part of the race. Those racing shouldn't have to pass you, you should make like you are not even there.
Backmarkers are indeed a part of racing. It's what makes racing, racing. Backmarkers can be used effectively in a race with another car if everyone follows rules and plays fair. Midrace joiners are NOT a part of racing and should make the attempt to be invisible to others who are a part of the race, including moving off onto the grass and shift-S'ing into the pits before the blue flaggers arrive.