Engine Management Light Problems?

I have a Audi 1.8 SE, recently the engine management warning light came on, my local garage plugged it into its diagnostic tester & it said it needed a new pre cat o2 sensor. The work was duly carried out & everything was fine for about 100 miles & the warning light came back on, back to the garage plugged into diagnostic tester again ,this time it showed the post cat o2 sensor needed replacing. Again the work was done but after 100miles the light was back on. Both sensors were replaced with NGK products. I was advised by the garage to avoid cheap supermarket petrol as this may be part of the problem. Is supermarket petrol cheaper because its of a substandard quality?. I would be grateful for any advice.