Contract error - was unaware of contract signed up to?

If a d&b contract is signed by the contractor in error (i.e. contractor wrongly assuming it was a traditional JCT contract) and most of the works have been carried out since, is it possible to have the error rectified?

Generally mistake will not affect the validity of the contract however much will turn on the specific circumstances surrounding how it came to pass that the contractor didn’t realise they had signed a contract with design liability. If the client or his agents deliberately misrepresented the basis on which the contract was being made then the contractor may have a case however if the contract was signed it would be difficult to see how this would be the case.

By agreement the parties may re-negotiate the contract however if most of the work has been done why would it be in the client’s interest to do this?