From what you have said, it seems that the hoist was made available to the Contractor for access purposes. The fact that the hand railing did not fit in the hoist does not make this a Defect under the contract, as defined under clause 11.2 (5), assuming your contract is unamended from the standard text, unless there was a stated requirement in the Works Information for the hand railing to be specifically manufactured so that it could fit into the hoist, to allow access.
This situation presumably makes access more difficult and, consequently, more expensive too. It seems that you have answered your own question about Disallowed Cost as well, so the outcome is that any additional cost incurred would become part of the Price for Work Done to Date and subject to an assessment of the Contractor's share under clause 53.