NJ> 19.56
Student neighbour thanks you for your effort and hopes you'll contribute to further questions in the future!!
Unfortunately this time, your answer is not relevant to his needs. HIS reasons:
1. DC voltage unrealistic in a domestic home set up.
2. Switch 1 unrealistic, as your diagram shows 4/5 terminals where in practice there would be just 3 (L1, L2 and C)
3. Only need basic diagram to show the principle of connecting the 2 single pole double throw switches without having to trace wiring.
4. Old cable colour coding.
Now Me LOL
I would add, the Junction box is superfluous and would only add to the cost of installation. The link below shows a conventional installation in line with practices today on new builds. I admit this also requires a lot of tracing to ensure it is wired correctly!!
https://ibb.co/R60SNHS
It is acknowledged that installing Up/Down stairs lighting, to overcome the Neutral wire termination issue, the design cannot get away from 3 core and earth cable between switches!!
Wishing everyone the very best of luck on the grand national :-)