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I shall delve into the suggestions, thanks for giving the pointers, most helpful.

Of course if you are using the FontDescription to change the font of a
row or cell you can use TreeViewCloumn.addAttribute to set the font
attribute of a CellRenderer.

column.addAttribute(aTextRenderer, "font-desc", 
ColumnNumber.fontDescription);

That is exactly what is happening in the code, well spotted. :-)

The problem is needing to change the FontDecription object to change the font size. In C++ it is easy to access things, gtkmm is hand constructed as a C++ API rather than being a GIR driven generated binding. With both Python and D, things are harder which seems to be a feature of the GIR-driven generated nature.

I have noticed though that my Python code is broken, I haven't got the D code working yet, but the C++ code doesn't amend the original FontDescription object as I thought, it makes a copy, amends, and then replaces the original.