I hardly do any miles on my Crossfire and have been told it must get serviced every year to keep the warranty up.
The second service is due in March and have been told it will be £400 even though the car will only have about 10,000 miles on it and it was last serviced at 6,000 miles.
The thing is yes, you don't have to take it to the main dealer however, they might not be so keen to do warranty work on the car if you have not had it serviced there.
For example, I have just had two new rear wheels fitted under warranty at a cost of £1,100 and they have also ordered me a new front wheel and a new seal for the driver side window.
The lacquer was coming off the wheels and there was a wind noise from the drivers window.
Having had such good service from the dealer I think I will be going there for my servicing.
Ok I could save £150 to £200 having the service done elsewhere but as it is only getting done once a year I don't think it's to bad.
Also I never do that many miles on any car I've had lately but I really think you should get the oil etc changed once a year.

Norrie.