Scaffolding was still a bit wobbly so went this morning and got more poles and clamps.  The trick would seem to be to make triangles, it is now significantly steadier.

Have cut a load of bricks for the corner, enough to get me to the top I think. And have built corners three courses. Will need to point one bit of the wall still over hand due to proximity to a tree.

Got the laser out to check levels and am about 5mm high at the back corner, not a bad tolerance over 12m !