sounds like that other guy would no better ive been a mechanic for only 5 years but i had a corsa that had the same problem and a few people told me to change the air flow sensor but i just replaced the spark plugs and it worked straight away... i would try that first before you go buyin an air flow sensor..
just noticed u said uve already done that sorry, probably will be the air flow sensor then, will probably only cost about 20 pounds to buy one but a garage may take upto about 100 off you