I think with modern car electronics there are all sort of tricks of the trade if you are in the know, at least the code reader may help avoid being ripped off or prove to be something simple.
I drove the car this morning and it seemed to have a bit more go when flogged so it may be something failing intermittently like the coil pack or air flow sensor which could be cheaper than a new cat, no idea how much they cost now but won't be cheap.
It could even be a bad batch of Petrol - it does happen.