Hi,
Why did the old cat die, if its not expected then perhaps your engine is kicking out too many nasties that killed the cat (an maybe have again)
Anyway its better to start at the front, with the engine, rather than the cat which is at the end. I wold look for a sensonr failure, lamda, tps, Idle air controll or even ECU. IS it a 1.8petrol?
Try taking it to an independant rover dealer as they can run diags on it no problem.
Also make sur the vehicle is serviced, air filter, plugs, oil etc. I ahd a golf tat had a cracked aibox and that maid the engine fail on emissions