Just because something is new does not mean it cannot fail.
Was it a genuine Yamaha rectifier/regulator or a spurious part?
Where did you buy it, many Chinese copies being passed off as genuine Shindengen units and are crap
My mate said the same.
Is there a way of testing one off the bike, as i still have the one i took off?
I only replaced it to eliminate it from my other starting issue which turned out to be moisture in the fuel tank, which i've now got sorted.
Not really, as you would need something to act as a Generator to supply adjustable 3 phase AC.
Simply way to check even though it is not fool proof, is turn on the ignition, put a meter across the battery and start the engine. Voltmeter will drop to around 10.5 to 11volts. Rev it to about 4-5000 check the reading on the meter, the voltage should start rising, now switch on the headlights, battery voltage will drop and start to rise again. If it fails this you will need to check the output of the Generator as explained in my earlier post.
However most Generators and Rectifier/Regulators fail when hot and it is better to this check after a trip.
Why not replace the regulator/rectifier with the old one and see how it goes.
I found that in the winter time when I go to work the short way in traffic most of the time, I would have to give the battery a boost from the optimate every 6 to 8 weeks, as it is never gets completely fully charged due to insufficient time above the 5000 revs mark