After a quite a bit of faffing around I eventually managed to get switchable graphics working in Windows 8 on my Envy 17 3002ea.
Following the recent release of dedicated catalyst drivers for my laptop, I decided to roll back from AMD's reference drivers (v9.0.0x). The new drivers list the f.08 version of the BIOS as a pre-requisite, so I applied the BIOS update. As a result the HD 3000 graphics has been rendered inoperable in device manager - the ATI card works just fine as before.
I don't appear to be the only person with this issue, see