My Greenline II is coming up on 4 months old and has passed the 9K mark, with service indicator saying it is due for its (1st) service in 500 miles.

 

I used to get 18K between services onm my 06 plate 1.4 TDI Polo on a variable service interval.

 

How am I not even getting 10K for the Greenline II (between services) on exactly the same driving conditions/route/style/frequency as the Polo?

 

Is it on a fixed service interval, and how can I tell?

I think you have answered your own question there, if it had been on variable servicing then it would not be asking for a service now.