Guys, thanks heaps for the overwhelming response!
To answer some of your questions...
The owner had an Inspection 1 undertaken by a dealer just recently - which included oil change and brake fluid.
The car is in immaculate condition inside, out and undernearth and doesn't miss a beat. In fact it actually drove better than any other z4 i've driven (smoother and more powerful).
The owner seems like a pretty honest guy, not at all pushy or over eager to sell. He's one of those people who is not really into cars, but bought it because he had the cash and loved the look of it. This was evident by his responses to a couple of my questions... Does it still have the original runflats on it? runflats, what are they? Tyres that literally run flat - see you don't have a spare wheel. Oh f*ck, I didn't even know. After popping the bonnet I asked - Does it ever make any unusual ticking/slapping noises when started from cold? I don't know mate, this is the first time I've ever had the hood opened. Made me laugh to be honest.
I've managed to get him down AU$3k off his already high asking price, but he will not budge at all from there. But even with the $3k (2kGBP) off, its still about the maximum you'd want to pay for such a car - so there's no real bargain to be had here.
Having spoken to some dealers and indys, they told me that they wouldn't be overly concerned about the long time between services.
You guys are right about another one eventually popping up, because one just did... except the dealer is aking AU$10k more than this car! AU$59,000.00 (39,500.00GBP) It really is ridiculous over here.