How do I solve the problem of re-occuring management light??

HI. I took my Vauxhall Zafira 1.8 petrol (51 plate) to garage to have the code checked as management light was on (although no obvious driving problems at all!!) and they topped it up with oil. After a few days it was back on so they fitted a number 2 oxygen sensor. It came back on again after a few days and I took it back yesterday but when I picked it up only the receptionist there and she didn't know what work had been done!!!!!! Today the light has come back on AGAIN!! I explained that it seems to be something to do with petrol as each time I fill up it goes off but I was ridiculed for that!! Any ideas?? I am sick of paying to have it re-checked...........................