Bingo! I've got it to work, just can't post a screenshot at the moment.

I realised that the black line, representing servo at centre, could not lie horizontal for flaps. It does for things like elevator, rudder etc which travel either side of a horizontal neutral. But when a flap servo is at centre, the flap is not horizontal, it is (roughly) half way down. When the flap is horizontal, like an elevator, it is not at centre but at one end of its travel. Somehow then we have to tell the program that the Control at centre is not horizontal but angled downwards, and that is done in the Oc box.

If you put in these values, it all works nicely:
Xs 8
Ys -0.5
Xc 1 (to rake the control horn back a bit)
Yc -2.5

Rs+ 45
Rs- 45
Rc+ 22.5
Rc- 22.5 (the angle of 22.5 either side of centre gives a flap travel of 45 degrees)

Oc -22.5 (this says the flap will be half down when the servo is at centre, this is the crucial bit that was missing before, a value of 0 says flap is horizontal when servo is at centre, so flap will travel up and down from horizontal, whereas we want up and down from half-way down!)

And now the diagram is exactly what we expect to see.

Harry