Hi,
Just following up the above as promised, so I removed USB from the equation and it behaves as follows:
If volume is anything above a medium level, when headphones are plugged in, the speaker volume dips a bit and sound is simultaneously delivered to the headphones and speakers. If you wait about 5 seconds or so though, it then successfully switches over to the headphones only. The sound delivered to the headphones is at a set volume, and level cannot be adjusted with vol + and - anymore. When the headphones are removed, volume to the speakers restores back to the default level which is saved on the board (in my case 20).
If volume is set to a low level, when headphones are plugged in, it switches over to headphone straight away with no delays. Again the headphones are at the same set volume as above and the level cannot be adjusted. When the headphones are removed, volume to the speakers is restored back to what it was before you plugged the headphones in - rather than reverting to the default.
As you see above, the headphone detection/behaviour seems to differ depending how loud the speakers are set to initially, but ultimately, it does work, just not entirely cleanly. I think the main issue is not being able to adjust the headphone volume at all. I've no idea if this was an issue on the stock firmware, I don't think I ever tested it.
When using USB audio, actually it does work to a degree, sort of similar to the above, but after a few unplugs etc, it can get itself into a right twist and sometimes you lose sound entirely. Also the quality delivered through the headphones is poor. So i've left USB off for now and just disabled audio dithering to remove any hiss from using the built in 3.5mm jack, and it's fine.