Car won't start ! HELP

Hi all, to cut a long story short... Had previous problems with the car, just had new engine put in and now more problems. When engine was put it car used to stall when trying to pull off from junctions. Was told this could be as the new engine is a manual and the car is an automatic, none the less was told it had been wired up properly. Car still ran ok until yesterday, got to the top of the road and car just lost all power, engine was still running but accelerator wouldn't work and were no revs. So had to push the car back home. Now it won't start, engine light is on, it does tick over but won't start at all. Had diagnostics done and showing code P0120. Something to do with throttle body ? And when trying to turn car over there is a clicking type noise coming from the throttle body. But opinion of another mechanic is the throttle body off new engine and old engine can't both be knackered ? (He swapped them to see). He suspects ecu ? Just dunno wot to think 2 mechanics telling me 2 different things! Help!!!