Blown turbo pipe and grey liquid on engine cover

To celebrate my car's one hundred thousand mile milestone I decided to have the timing belt changed at a local garage.

After about an hour of collecting the car I heard a pop and lost most of my power.

Another motorist stopped and confirmed that my turbo pipe had come off. I rang the garage and he almost too quickly agreed.

When I looked under the bonnet the engine cover and other plastic parts were covered in a spray of a grey/brown substance. This has stained the plastic and will not come off. When I rub the substance it becomes shiny almost like graphite from a pencil.

There was also water, oil and a greyish gunk around the filler cap on the engine.

I sent the attached photos to the garage and he says the stain is from a "cleaner" they use on the engine. It is obviously not a cleaner as its actually stained my engine cover.

I am taking the car back to him today and would really appreciate some help in diagnosing the problem so I can force him to accept blame and repair the damage.

The guy that helped me on the side of the road thought it could be a gasket issue as there seems to be water and gunk around the filler cap.

How could fixing a timing belt create these problems?

Thanks for your help