How long ago was the MOT? was it using a lot of fuel prior to that?
If the car was overfuelling badly, it would have been picked up on the CO test during MOT as would any Fuel leaks.
Was the MOT done at a reputable Centre?
Your Corsa should be averaging 35-40mpg at least, but is currently giving on 24-28mpg.
You have several routes you can go down.
1. Is there a fuel leak from tank/pipes etc, if so you would smell it
2. Engine management fault. Has your engine management light come on?
Could be an ECU fault or one of the many sensors, the one of the most common faults is the Fuel Temp sensor which can cause it to overfuel and stall when warm? If not that could be a number of other sensors.
Other than that could be anything, mechanical fault, faulty injector, clogged air cleaner etc
I'd go get a CO test done, and if your management light comes on a Tech 2 test at your local Vauxhall dealer
CJ