Well good news and bad news. As a principle it works very well- i.e. bluetooths conneciton was fine, powered from brown (earth) and purple/white (swicthed 12v) from the OEM wiring harness. With a little trimming of the plastic edges the 10 pin IDE fit nicely in the OEM CD changer socket, and transferred the audio correctly from phone.
However the quality of the audio was not good. Very distored.
So to test where the problem lay I hooked up the 12v power to a molex connector in my PC and used the proper RCA out to my amp (which is pretty neat in itself - a bluetooth amplifier!) but still poor audio. So next I unsoldered my additional IDE cable so see if that was generating interference. Still no good.
So my conclusion is either a faulty Bluetooth to RCA adapter, its a poor quality bluetooth audio chip, or I have damaged the pcb or chips when I soldered the wires (perhaps too much heat) but I doubt this as the RCA connecters are a far way away from the chips.
I am going to try and get a replacement adapter from the ebay seller and try again...
Otherwise this would have worked flawlessly in terms of connectivity!