Vauxhall Tigra new cat, am i doing the right thing?

Hi everyone. I sent in a problem a few weeks ago under "Idle and starting when hot" and recieved advice on my car juddering and hard to restart when hot.
Just recently the problem has become worse and with net searches it pointed towards the EGR valve. Today I took it for a diagnostics check to make sure before i purchased one, and at the time the car was juddering as it does when hot with loss of power. The check showed only ONE code (24. Vehicle speed sensor) which is an ABS fault. So we asume the engine is not at fault and had only had a service a few weeks before I purchased it.
The mechanic said its a difficult one but could be possibly 1 of 2 things. The ECU has lived its life and not controlling the engine or the Cat is blocked which would send exhaust gases back causing the engine to choke. When hot there is a strong sulphur type smell which he says indicates the Cat. So to see if it is the ECU or the Cat I am risking the cost of having a new Cat fitted tomorrow to see if it is the cause.
The car had a new front and rear exhaust just before purchase which makes me think someone else may have had this problem thinking it was a blocked exhaust but didn't want the expense of the Cat
Question is, if the ECU is faulty would it have given the code for the ABS fault and not the engine?
Could a new Cat be the solution to my problem and it isn't the engine at all?
I have had no dash lights come up until yesterday when the engine check light came on for about 8 seconds and then went out after a speedy ride and slowing down to a village, but shortly afterwards the car engine stalled when slowing down behind another car.
Could it be the gases went back and disturbed the sensors making it think there was another problem so bringing on the engine check light?
Anyone out there got any advice for me please, or any other ideas or can you give me confidence I we are perhapes taking the right moves. Would be grateful for advice.
Thanks, Fenman 3