Thanks for the replies guys. The reason for asking is that I was sure mine always used to idle at 700-800rpm but at the moment sits at 500rpm. At this rpm there is quite a lot of vibration in the cabin and the car feels sluggish during normal driving (still goes well when given the boot).I recently had a problem with the battery going flat so it sat unused for a couple of days with the battery below 5v before it was replaced. Would this have caused anything to reset to default?
You should really scan it for any fault codes. Sometimes a spurious fault code is generated because of low battery voltage (electronic chips don't like excessively low supply). If you have a fault code then delete it and drive the car normally whereby it will gradually re-set the idle speed.
There's also a manual re-set procedure I've heard about, something like inset key, turn ignition on but don't start engine and floor the accelerator pedal three times and release within 10 seconds then start engine and run normally. Worth atry.