I always did wonder about the green symbol saying "Locked", however, despite suspecting is could've been the cause, you inspired me to go have a fiddle with some of the options and hey ho, I'm in business!
I just clicked "external spdif" which I circled in the picture, then as displayed below that, the "locked" symbol changes and I can suddenly play with the sampling rate as I please. For some reason when I switched it to higher sampling rates I could hear a hissing noise which did bother me.
However, after flicking it back to "internal" as circled again the hissing suddenly went and I had the sampling rate I wanted. It switched back the first time, I found this was because I had Winamp open so I closed it and tried again, but succeeded! (I guess it'd be the same with any audio orientated program, although I did leave my browser open and it worked, but nothing with audio was actually loaded, so yeah...)
I doubt I'll be using a 192khz sampling rate for recording since it'd be a bit overkill, but we'll see...
For anyone else who may have been having this problem, hope it helps, but thank you brockorama for the help!
I still get popping with the buffer size, but no idea how to get around that...