Thanks for getting back.
@bentraje I've installed the driver, and that does seem to have sped up the rendering in the picture viewer, that's brilliant thank you!
@MJV as far as I can see the interactive (RS viewer) and final rendering (picture viewer) should match if they're both set to bucket. But I'm still very much trying to get to grips with it, so may well be misunderstanding a lot of this! (I've attached an image of the main sampling settings I'm using).
There is a very slight difference between the renders (quite hard to notice), so you're probably right about the color space issue.
Still, the picture viewer is much faster than it was, now I'm using the specified driver, so thanks again for the help.