A 1990-registered car does not need to pass the 'catalyst' emissions test even if a catalyst is fitted. And you would be free to remove it if you wanted to. The figures you have quoted would not quite pass a 'catalyst' test but are well within the permitted limits for a 1990 car. If your tester is not aware of this, try a polite reminder. If that does not work, I would suggest taking the car elsewhere (but have the rusty sill repaired first, because that will fail wherever you have the car tested

I mention 'dishonest' because if your car does have a catalyst fitted (which does surprise me for a 1990 Rover, I thought they were not available until 1991) that catalyst will have a very high scrap value. So if you have it removed, I would suggest you insist on keeping it!
I should point out that I am not a qualified MoT tester but I do have a good working knowledge of MoT tests and most of the information is freely available from 'gov.uk': it just needs a bit of patience to find it
