The "hotspots" are the pre-combustion chambers of an indirect injection
diesel engine. The shape of the chamber inside the openings is crucial
to the igniting of the fuel.

It is very common to find cracks on these and as long as they are small
and do no run to the edge of the hotspot you should be fine. More
importantly check that the hotspot is not loose.

As it is a 2.5TD have a very close look at the top of the pistons and
see if they are cracked too.

Good luck