I did not have much luck so I regret I can't add an 'answer' to the question.
I found going to the various Microsoft sites was like trying to get a glass of water out of Niagara Falls and, at one point, I came across a recipe for pancakes. Nor did reverting to NC 2008 permit an install.
In summary, the Microsoft documentation said the 32-bit NC is installed when installing the 64-bit NC. This may be so for earlier releases but it is not the case with Windows 8 and SQL Server Express 2012.
MS SQL Server did install NC but the 64-bit version - which is not an option for PowerBuilder 32-bit.
In this respect, as Scott Morris graphically demonstrated, even if I could install the 32-bit NC for SQL Server 2012 on Windows 8, the PowerBuilder 12.5.2 Database Profile Setup is hard-code - no doubt for a reason - to only allow selection of NC 2005 or 2008.
The fall-back option, as suggested by David Peace and Roland Smith, was to install NC 2008. On running the MS .msi download, I got the message, "Installation of this product failed because it is not supported on this operating system..." - the same message I got attempting to install NC 2012 32-bit.
This was unexpected as I have MS SQL Server 2008 (with NC) on a Windows 8 machine. NC, in this case, was installed from the SQL Server installation and not an .msi download.
(Installing MS SQL Server 2008 just to capture NC is not a viable option as neither I nor two technicians could manage an install of SQL 2008 on the computer in question - an Asus combined tablet and laptop with a i.7 processor - hence my move up to SQL Server 2012).
So it's back to the future and I've set up on an old XP laptop so I can get on with my work.
A dilemma remains, however, and it's perhaps a dilemma for other developers, which is what to tell a client if they ask, "We're moving to SQL Server 2012 (or 2014) and Windows 8. Is you application compatible?"
PB 12.6 may be the answer to this question as it has 64-bit. But it seems implicit from what I've read that you can't migrate an application from 32-bit to 64-bit, this implicit notion coming from a statement that said it would be necessary to develop in both 32-bit and 64-bit - two separate applications. Stated differently, as I read it, you couldn't develop in 32-bit and migrate that version to a 64-bit version. Hopefully, I'm wrong on this.