Engine damage after replacement of the cylinder head seal

Hello, I have a big (and expensive) problem and I am looking for advice on the following: At our Citroen Berling (EZ 05/00 – ca 155000 km) a Citroen-Service-Werkstatt cylinder head gasket including water pump, timing belts and tensioning roller were changed. After 5 days / 600 km the moter suddenly began to “nag”. Diagnosis of the workshop: crankshaft and connecting bearings defective (strong Riefenbildung and many chips in the oil). A reason for this new damage could be Not clearly mentioned to me, also a causal connection with the previous repair was negated and should not be proven. Now a replacement engine (or a new car) is necessary. Do I have to accept the notice of the workshop, or is there the prospect to accept the connection of repair and damage and hope to guarantee the workshop? Who can provide me with tips on the further procedure ??