Picked the car up from AV today, problem solved! Here's the full story in the hope that it helps someone else that finds themselves in my position.
I had the car recovered a few weeks back, not wanting to chance 50 or so miles and the risk of losing all but one gear along the way... a pic of that will be going on the low-loader thread shortly!
With the SD3 plugged in she was showing code P2637 - the gearbox's process of managing the engine rpm downwards during gear upshift, as I understand it, wasn't being allowed to happen given the engine wasn't able to support this request at the time of the issue. This suggested to us that something is happening further upstream i.e. within the engine to cause the transmission to throw a code... which kind of figures given the ZF is known to be a generally reliable unit.
The check engine light/dashboard warning messages would come and go intermittently, so the challenge would be to capture this issue upon occurrence and see what other error codes are reported at the time.
After two long drives, nothing. The issue happened to me about 45-60 mins in urban, stop/start traffic, and we tried to replicate this to get the error to resurface. However, the next day when AV was moving cars around his workshop, with the engine running and car stationary, the dashboard does it's Xmas tree thing again... and because the gearbox wasn't doing anything at this time we have even more reason to believe the transmission isn't the root cause.
What we did see was an error relating to the air flow sensor and, when taking the air intake system apart, it was clear that there was evidence of dirt ingress which seemed to be resulting from bad fitment of the air filter... apparently dirt getting in is what damages these sensors. These are stills taken from a video AV sent, please excuse the centre play icon...
Here's the chewed up air filter, with chunks missing out of the lower edge.
With the air flow sensor fitted and a few long journeys under her belt since then, I'm pleased to report all seems well again
I still can't get over how vastly unrelated a transmission fault is to the replacement of an air flow sensor, although I'm massively grateful it wasn't anything major...