Just wanted to give an update on this - took my bass to a luthier and they identified a slight ski slope at the bottom of the neck. Took the frets out, sorted out the bump and re-fretted the neck. After putting the same La Bella strings back on, the buzz reappeared. The fretwork has certainly helped as it now plays much better in the higher register, but the strange sitar like buzzing from the G string is still present. The luthier did a great job and is very experienced but even he couldn't get to the bottom of it. He suggested it might be an issue with the strings themselves.
Went down a bit of a rabbit hole and discovered that this is an issue that some others have experienced with La Bella strings - particularly the G string. Sure enough, if I string my bass with different strings then the issue disappears. Furthermore, putting the buzzing string on another bass creates the exact same issue - so I am certain that it is the string.
I have tried a couple of sets of La Bellas and unfortunately all have had the same issue. I've been in touch with the shop I purchased them from, as well as La Bella themselves, and both have shown great customer service - should have some replacements coming soon.
I'm hoping that I have just been unlucky here, as I've never had any issues with La Bella's in the past.