petrol light on

[tick]

the petrol light in my car has came on even though there is quarter tank in it, any ideas what the prob is likely to be? Also idiling has been rougher than normal, kinda spluttering this linked?

 

[KDF]

My petrol light did this once, put more petrol in as a precaution and the light went out again, its never done it since (about a year ago)