Shot in the dark help, anyone knows about Mercs

My Son has a 15 plate CLA 180 with 28K on the clock. When he bought the car i noticed residue around the coolant header tank (Not cap but between the join below cap and main tank like house and roof). The garage said it needed to be investigated and they did saying no fault found and they cleaned the header tank, all good for a couple of months.
Car goes into Merc for health check as my son reported a knocking noise from front of car which was diagnosed to be an engine mount which was replaced under warranty.
They said it needed rear brake pads and they wanted £145 an hour plus parts so my son took it to a Merc Indie who did the job for £150 fitted. The indie noticed a major split in the coolant header tank and said the engine didn't sound right, he instantly picked up on this also said the colour of the brake fluid was a shade he would expect on its next service. My son drove the car home and Merc dealer arranged for a technician to visit the house it was towed away to a main dealer, they have replaced the header tank, thermostat and another 6 hrs to replace the water pump as it was a bigger job than what they thought, they did an engine compression test which they said was good.
As i have basic knowledge of cars what would cause the tank to crack as the Indie said they are made of good quality plastic.
He did say something about the engine noise being a con rod, i have also heard a small head gasket leak could cause the tank to fail.
The car is getting a full check on Monday by Merc, just worried as the car wasn't cheap any ideas as i said just a shot in the dark

many thx Gos