1. The screenshot from Chris Tringham shows a structure of the calculation, where can I access that structure?  (I am presuming there is some way to edit it once I know where to look at that)
2. What will be needed achieve the layout I expect showing common stock as its own line under Equity, as per the cheat sheet (and what I would expect on a set of accounts anywhere)?
3. Whatever those steps are, will they something all users are expected to setup, or is this some error in the UK defaults as it feels like this should already be in place for me?

The code range doesn't really matter, it's the formula (and labels) on the Balance Sheet that control this.

In v17, the "Retained Earnings" line is the sum of the balances of all the Equity accounts:

I assume your new account is of type Equity, so indeed it will be included in Retained Earnings.

You can change the structure of the Financial Reports in the Settings menu (this is v18):

Note that in v18 there are two lines for Retained Earnings, current year and previous years but it does not separate out the Common Stock.  In v17 there is one line.

Unfortunately, Odoo documentation is sometimes like that - the "Cheat Sheet" is helpful but doesn't quite match up with current system!