Update: I installed and ran Everest + found that the name of the chipset is ATI RS400. A bit of googling revealed this thread:
http://forums.majorgeeks.com/showthread.php?t=109221
where someone with the same motherboard seemed to have the same problem (I also had several other devices marked in device manager until I installed the catalyst drivers), so I did what worked for him and installed the Realtek High Definition Audio Driver 1.84 from
http://www.majorgeeks.com/download.php?det=4902
Unfortunately, it seems to have made no difference whatsoever
The person in question also said they had the Audio Adapter ATI SB450 - High Definition Audio Controller on their system, which I've found no mention of on mine, so that presumably is why it didn't suit. Anyone any ideas?