Crank sensor is an easy and cheap fix - bit awkward to carry out as access is hideous unless you are a double jointed midget with extra long arms.
Happened on mine whilst waiting for an MoT, no warning, just refused to start - read low loader thread for pictures!
Don't rely on the dash voltmeter it's not very accurate (well mine isn't!) It never reads more than 10volts on start up with a full battery (I keep it topped up with an optimiser, when it is not used on the motorbike)
Starter motor is also a good call they slowly disintegrate internally causing progressively slower cranking, eventually they throw themselves into a billion bits and jamb solid - but only when on a parade lap of Brands Hatch or some other important occasion.