I had this problem with my G. As Haysey and giorgio said check the battery with a multimeter first (should get about 12.7 for a good battery, mine had 12.53), or get a garage to do it for you.
If (like mine) the battery's fine, follow haysey's guide in the FAQ and clean up all of the earthing straps and battery connections. Some of them were really corroded on mine, preventing the large amount of current required to crank the engine to get through. Having done that this weekend, fingers crossed, it appears to have sorted my lazy starter woes!
Ruling out both of these doesn't take long and its free!