OK, the results are in...& it works!
I'm not sure how easy it will be to figure out the exact time that each piece of equipment completed its start sequence. The modem & router are in one room, and the NAS & Pi are in another, and because the Pi has no on-board battery the times reported by DSLstats are wrong by several minutes until the Pi picks up the network time server.
However, I don't really mind. The data is being saved on the NAS...which was the primary aim of the project.
If I really want to know the detail of what is happening in the first few minutes of the modem starting I'll keep the monitoring equipment running throughout. But right now I have other priorities.
Thanks everyone for your help.
George