Did you activate these fonts in the OS?
Also, Photoshop will remove fonts from the list if the font files are obviously damaged (the font engine in Adobe products does extensive error checking).
I did, yes.
Do the fonts work in other applications?
They work in Excel, which is where i will be copying and pasting from.
Right, i'm not entirely sure what i've done here in terms of the event sequence and why one of these fonts now seems to function correctly, but i will list what i think is the order of steps i was experimenting with:
1. Go to Start > Settings > Control Panel > Regional and Language Options.
2. Select the 'Languages' tab at the top of the new dialogue window that opens.
3. Select the 'Details' button that appears on the next screen.
4. Added Japanese to the Installed Services on the next screen (Click 'Add' and then select 'Japanese' from the dropdown menu to do this, i left the 'Keyboard Layout / IME' exactly as it was before).
5. I then selected the 'Advanced' tab at the top of the main 'Text Services and Input Languages' dialogue window.
6. I checked the first box on this screen which reads 'Extend support of advanced text support to all programs'.
7. Apply these changes, close all windows, restart the PC, and this appears to have done something. Incidentally, the font that did seem to render correctly hereafter was titled M+.
8. Also noticed that the input language is now currently set to EN (english) in the language bar and not JP (japanese). I had thought that it would be set to JP in order to access the japanese font characters, but the font characters do seem to showing up.