Travel Republic have something of a reputation of doing this, but in law, your contract is not with them or the airline. As this is a charter flight, your contract must be with an ATOL holder, that is someone authorised by the CAA to sell the seats. Travel Republic should be acting as their agent and must, in law, have passed on the invoice to you. That is the company to contact.
There is no 12 hour 'law', within the ABTA Code of Conduct, and who knows whether your contract is with an ABTA member or not, a suggested time change of 12 hours is considered 'material' but the real issue within the code is 'inconvenience' and this is certainly inconvenient. If TR do not offer to pass you a copy of the ATOL Confirmation, which you may find is a different price to that which you paid, contact the CAA help desk on 020 7453 6424 and they may be able to help