Just brought a Macbook Pro 15" Unibody 2008, its trackpad works all well apart from clicking, for a click to register you need to push hard. Is there anyway I can fix this or is it a physical issue that cannot be fixed?
It's certainly not a physical issue that cannot be fixed because I have that same laptop and the trackpad click works fine. If you don't want to take it to the Genius Bar at Apple, then I suggest you read up on how to take it apart, clean it and reinstall it properly or possibly replace it - ifixit.com has great step by step tutorials and videos on how to achieve that.
You could replace it or change the setting to 'tap to click' in System Preferences. I rarely use the physical click because I don't want to wear out my trackpad.