Land Rover Repair: P0420, p0420 catalyst system efficiency below threshold bank 1, threshold bank

Question
I have a 99 Disco 1 CD 4.0. I have heard rattling from my catalytic converter area for the past year or two. it comes and goes. right now it starts to rattle when the engine is cold and is louder when I accelerate. once the engine is worm the rattling goes away. My check engine light came on a month ago then went out a week ago. it came back on a few days later. I checked the code and it is P0420:Catalyst System Efficiency Below Threshold (Bank 1). is it likely I need a new cat converter on bank 1? is bank 1 on the drivers (left) side? should I check replacing the 02 sensors first since that is cheaper? it this likely the reason my light came on and went off a month ago? why does it go off? if I continue to run it with this error will I have any issues other than getting it inspected?
thanks

Answer
The catalyst is a one-piece unit; left and right are together.  I suggest you change the o2 sensors at the same time as you change the catalyst.

There should not be any other issues from running like this, except for failing state emission test as you noted.

I suggest the aftermarket catalysts from Atlantic British or Rovers North as a less costly alternative to the dealer part.