Having had a quick look over my matrix code last night, and a re-review of the stats, I’m pretty sure that code is good.
So today, I decided to take a different approach: just cram in as many flights as possible, and look for common behaviour.
Each flight took off from my favourite patch of daisies in the lawn, and used identical configuration. Each flight drifted, but the fore-aft drift seemed entirely random; the port-starboard drift seemed more consistently to starboard, notably in complete contrast to the wall-collision on the port side yesterday.
So absolutely no conclusions can be drawn as yet, but I plan to proceed along the same lines later once Phoebe’s batteries have charged in the hope that some consistency of behaviour starts to emerge.