2004 Vauxhall Corsa loud clicking/clacking sound from engine when driving

hi guys i hope someone can point me in right direction as car has had 700 quid spent on it and its still not right. i bought the car less than a month ago and had it serviced 2 weeks ago. it was running fine before the service and hadnt had one since 2005. it came out of the service awful, with this loud clacking sound from the engine that sped up when accelerating (sounded like a diesel). i took it back and was told it was the tappers. had new timing chain, new tappers and alsorts of other things done to it to get it running ok again as the car was undrivable. now its supposobely fixed but this sound keeps coming and going, one minute the engine quiet the next its clacking, and the the tickover seems to be loud hwen stationery. had diagnostics run onit but they couldnt pick anything up, but diag was ran on it also when the car was undrivable and it didnt pick anything up then. any ideas? thanks people.