VW Polo 1.4cl 98. Lack of acceleration problem.?

It does sound like your valve timing could be out, worse scenario is is a valve is bent in the head causing low compression, i would get the valve timing checked and then have the compressions re-checked, if the cam-belt snapped or jumped a tooth when the alternator drive belt went through the cover it would bend a valve, clearing a trouble code will not make it run any better, my guess is you have valve problems, Do you know the garage who did the work?, sorry but if you dont its possible there trying to fob you off, after all you have obviously paid them for half a job, take the car back to them and demand they put it right, or tell them you are not happy with what they have done and to find out whats causing this fault, if the engine has been run with the belt timing wrong or the belt snapped i am 99% sure you have a bent valve or two, good luck