Pulsing Throttle

The Vrs has developed a wandering throttle. At tick over when warm it starts to rise to 1500 RPM, it then dips or stays on till you blip the throttle.

At first it would only happen once or twice in a 20 mile drive to work. Now its more predominant. Sometimes getting up to 2000 rpm. Never an issue once the car is up to speed.

Compressing the clutch doesn't alter this. Anyone had this?

I'm thinking

plugs,

leads,

fuel filter to start with. But any areas that I should go to first. (all of the above are due to get changed)

could a 9 year old battery do this? I've had this before on bikes, the battery starts to go and similar symptoms. But I am unsure if this would make the blindest bit of difference.