Hi all,
So my original problem was, as you can imagine, the battery refusing to charge. Following the manual, i performed a number of tests with the engine running at 5000 rpm. While running the bettery voltage dipped to 11.5 v, keep in mind this battery had just been fully charged from the mains and is brand new. Generator was good, little resistance on the coils and all 3 were outputting around 75-80v. Finally after testing the rectifier i believe i found the fault, all the diodes seems to be functioning normally with only 600ohm resistance and all had infinity on the return. however when i tested the resistance between the positive and negitive outputs was only 900ohms, a quarter of what the manual claims it should be. Finallg when i tested the output voltage of the rectifier on its own, it came out as only 0.5v.

So i've order a 2nd hand rectifier to replace it however i was woundering if there are any common or preventable reasons why the rectifier fails?

Holy jeebus Christ. You're on a motorcycle forum, I play with electronics as a hobby and I don't think I've ever tested the bike this way.
OK fine, look into a schottky diode rectifier if you're so into these, a stock R/R aint gonna float your boat.
Cool.
Buddha.

What causes them to fail is heat, same as what causes all passive electronics to fail. Diodes just fail eventually given enough heat.