i had something similar with my old astra, had the cat changed and it struggled through an emissions test, and the engine management light stayed on constantly.
My mate was a vauxhall mechanic at the time and he told me that you need to wait a few hundred miles for the carbon buildup to happen enough for the cat to be effective.
What he did which remedied the light was he disconnected my battery for a while and reconnected it, thus clearing the error codes on the ecu. make sure you have your radio code though!
I'm not sure if the same applies to your case; this was mine and my car was fine afterward