THE SOLUTION!
Anyone who has read this thread from the start will know the issues i've had! and well today, i solved it! and the problem? the HDMI cable. The one i bought from Amazon was listed as HDCP compatible, turns out, NOT.
So I bought a cheap one from Argos that was listed as HDCP compatible and plugged it in. Lo and behold, ATI HDMI audio Ready! The jubilation was excessive to say the least! xD
I've since done some testing, and for any others with the problem, i have found:
You need the supplied ATI HDMI - DVI adapter; supplied with most cards, bar the XFX version apparently. For the 4890, and I imagine the rest of the 4xxx series, the adapter should have either: Rev. B / 6140063500G / 6140063501G stamped onto it; an ATI logo also stamped onto it; and be a grey colour.
Now the pin arrangements (don't worry about the rest of the adapter for now):
The Rev. B and 6140063500G will look like this:
(Thanks for the image Clairvoyant!)
The 6140063501G will look like this:
(Taken from internet)
I have both of these adapters, and have tried them both, with the old and the new HDMI cable. Neither would work with the old cable, however both now work with the new one! Therefore I can conclude both will work if you only have 1 or the other. (But I reiterate, they MUST be the ATI official ones!)
Enabling the audio over HDMI is then simply a matter of going to playback devices and selecting it as the default (if you wish to always default audio to the HDMI) or from within your individual software set it to the output your require in the audio settings.
Thanks to everyone that has helped me to get this problem sorted and fixed, i'm extremely grateful

especially Clairvoyant and Kingy666!