If it's a Rover 75, I guess it will be well before DPFs? Can't remember when they stopped making them? Assuming no DPF, you MIGHT just get away with some "hard" solder, but brazing would be better. MIG welding would be my preferred option though. The outside of the steel might be aluminised (or even zinc plated), so that will have to be ground off. Just be careful though, my dad's Landrover Freelander (2001 vintage) had a "stainless" system as original equipment. I think it was 409 grade, so it did rust a bit, but not much. I don't know if that was common on other Rover group cars back then? Assuming good access, I wouldn't bother taking it off the car.
Years ago, some friends had a Polo with the same problem - and otherwise good rear silencer with a hole in the rearmost end plate. I welded a patch on it and it was still like that, 2 years later when they sold it!
Years ago, some friends had a Polo with the same problem - and otherwise good rear silencer with a hole in the rearmost end plate. I welded a patch on it and it was still like that, 2 years later when they sold it!

