Just thought I'd drop a quick update to anyone interested...
Well I tried my iPhone in another merc (an A45 actually!) and in a BMW (sorry) and still couldn't here anything through the speakers.
I then borrowed an iphone 5 off a friend and would you believe it, it worked first time!
Definitely a problem with the phone I reckon.
I then took it back into Apple (bearing in mind they only replaced it 2 weeks earlier) and explained my issue. They knew straight away from my explanation that there was a bluetooth hardware fault so they agreed to replace it yet again for me...result
I connected it to my car and it worked first time.
If anyone else has similar issues then maybe you have the same problem as mine.