Differences between a 12 & 24k service ?

[roshambo]

Can anyone tell me what more gets done to the car at a 24k service over a 12k service ?
There is a huge difference in price so curious.

Cheers

Ro.

[roshambo]

Thanks guys, yup plugs looks like the main big difference.
I ask because my car has only done 18k miles in nearly 3 years, so I guess the plugs don't need doing just yet ??