Many thanks for the replies, sorry it has taken me a couple of days to get back.
In respect of the terminal 50 measures, on the Ch.C on the latter trace, this was taken by probe at the terminal 50 wire adjacent to the starter motor itself, not at the control pin on the relay. As it happens, I'm in the process of making a relay extension harness that will allow me to measure the voltages / currents directly under the starter control relay in the cabin. The reason I wanted to do this is that changing the relay (original one is marked 643, and appears to be the VW original) for a good, but secondhand, Siemens branded 40A relay, did seem to make a slight difference to the oscillations on starter current as the solenoid clicks in. I couldn't be too sure whether it was just by chance that this was the case, so I did an A-B-A test, and convinced myself that the replacement Siemens relay appeared to control the solenoid a little better.
As I don't presently have a new relay to try (it is more difficult to get quality relays with diode protection, than cheap&nasty relays from dubious sources), I am awaiting a new batch. The VW one that I ordered was the wrong type - a more typical 5-pin unit rather than a convention 4-pin unit as-fiited to this car.
I'll post some results when I have them. I am sure that you are all correct that I'm oier-analysing this a bit, however I have a chance with this car to do some experimentation at my leisure, and I do want to ensure that the customer in this case is not going to have further issues in the future.