Assuming this is an unamended JCT liability for failure or discrepancies in the Employer’s Requirements does not rest with the contractor (this is usually deleted however and all responsibility placed on the Contractor).
There is no simple answer to this I am afraid. You need to start with the defect, identify what it is, then who it arose and what actions could or should have been taken to deal with it. The difficulty for the Contractor will always be that he should have spotted the problem and notified of it before continuing with his design, that is, the defect should never have materialised. A reasonable starting point may therefore be that it is never going to be more than 50% the fault of the employer (the worse the fault the more obviously the contractor should have picked it up). Beyond that very broad starting point it really did become entirely reliant on the facts.