A quick but proper diag session will rule out any fault with the management side of things and a gas analysis will confirm a timing fault as being likely or not. After that, you need to consider it being a mechanical fault, so we check timing/dephaser condition first and then carry out a compression/leak down test to see if the lump is just scrap. Believe it or not, not all faulty dephasers rattle like a diesel yet they can still cause the timing to be incorrect... These cannot be re-timed and need replacing.
You'll never find me naming and shaming as its a practice I don't agree with but we do see a lot of cars which need minor correction. The difference even a small error can make on these cars is unbelievable.
Any car with pretty much any age belt can be corrected but if we're looking at a belt that's 4yrs old or more, It might as well just be replaced.
Mick