Well if your mechanic friend thinks its an injector, which it could be, just pull the injector clips one at a time. If one of them has no effect then it's that injector or plug. What does it normally idle at? has that changed?
Ticking could just be the valves. If you have a factory ZX turbo you probably have the P90A head with hydraulic lifters. That could be a sign. Double check on the head stamp for us, or if you know it has solid or hydraulic lifters let us know.