OK ... decided on way forward ... thought I'd post solution for others.
I have bought 150 x 50 x 910 mm precast concrete rear pavement edging.
These are square edged, and being 910 long should be much easier to get accurate in terms of plum & level, needing only 4 to do full side instead of
16 bricks. This will also add 150mm to effective eaves height ... which people advise is a good thing, and being 50mm thick does not take up any room inside greenhouse.So that is the job for tomorrow ... not done any trowel work for a few years... so will be interesting.