When braking I get that pumping feeling like I am repeatedly loosening/tightening the lever.

From this I am pretty sure this is due to a warped rotor.

Before I start working on this is there an easy way to check if just one or both front disc rotors are affected and if only one is it OK to replace just one or do they need to be replaced as a pair?

Finally is there any other reason that this might be happening other than a warped disc?

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Bike: 2008 XT660Z Tenere with various bits and bobs added or subtracted