If the manifold bolts sheer whilst removing to do a headgasket they will repair it but charge you which is fair enough as the job is related and they need to remove it to take the head off.

If the garage however was doing a headgasket and destroyed a starter motor (or in my case a gearbox swap that smashed my alternator in to its component parts ) then they should repair it with no charge to yourself.

Somthing about "Failing in their duty of care" I believe.