One other thought,
When was the air filter changed last? A blocked filter can cause a CAT fault. If its not been done recently or was a cheap one it might be worth replacing it.
There is no question on the MOT, if the engine light is on the car will fail.
I did have to highlight that when I turned the engine on I could always hear gushing liquid. Someone suggested that as that had only started since they installed a new radiator and fan for me, they hadn't bled the radiator properly. This didn't seem t effect the running of the car, but could that issue have adversely effected the catalytic converter?