Is the battery negative onto the gearbox and is the connection clean and tight?
BE CAREFUL Don't overtighten that bolt it's just into the alloy
To see if you have an earth problem?
Take a decent car jump lead and hold it on the negative and the other end clamp to the exhaust clamp if it's clean and shiny Press the button if you get full power starter then you need to sort the battery earth out
If Not try the same BUT you need to be careful as you are going to use the jump lead on the upper starter connection and battery positive so watch you don't ground the clamp on the engine casing (few bits of insulation tape on outside of the jaws or a bit of rubber hose This lets you know if there is a main power cable break or fault if someone pushes the button and the starter is full power (you'll be too busy holding the clamps to press buttons!)
Other than that as Kenny says the starter relay is the main suspect but is it the starter that is clickin or is it the relay??
