1999 2.5 petrol manual estate
I know left hand cat rattles, might this be the cause, or could it be something else?
The rattle from the nearside cat will likely be the heatshield on the downpipe from the manifold to the cat. They all corrode and get loose and move on the pipe. Usually both sides are affected so I guess your offside cat is newer, or maybe the offside shield has fallen off.
Folks clamp them up or otherwise jam them to stop the rattles. Or I've read reports on here that it's OK to cut them off altogether. Or live with it.
Get underneath the car, engine cold, jam the heatshield to stop it rattling, thump the cat with the side of your clenched fist. If you can hear a rattle from inside the cat then the matrix is broken and the cat will have to be replaced. However if that was the case you would have other symptoms such as sudden loss of power or rough running for no apparent reason. Maybe after hitting a bump.
I don't think blanking the EGR would fail the MOT, unless maybe the EML was on during the test.