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I can't understand why 2-methylpropane has/needs the 2 in front of it?
It appears to me that if you were to add the methyl group at the first or last carbon on the chain then you get plain old butane and so defining 2-methylpropane is unnecessary when it could just be methylpropane.
Have I missed something or is this just a convention? Apologies if it's a really simple question; it's my first ever time studying organic chemistry.
You are correct. It is unnecessary to include the 2 in order to provide an unambiguous name. This compound is also often called isobutane which is an older name that is still in common usage.
Yeah. You're right. If we add methyl group(-CH3) to first carbon(or last carbon) on the chain of 2-methylpropane(isobutane) you would get butane.
The below figure is butane.
The below figure is isobutane or 2-methylpropane. 
