This one just gets weirder. It ended up at the local Ford dealership after I failed to find the fault. Anyway, they told the customer that there is nothing wrong with it, but did confirm that this particular model of Transit came out of the factory with this "anomaly". Anyway, the driver is still refusing to drive it, so it appears to be stale mate at the moment.
This sounds like a software issue to me. Surely, if the speed threshold is raised at the point at which the ABS actually cuts in, the fault would be cured....