Are they an approved repairer accredited to the MVRA? If so then you could report them and they may lose their quality approval. If they claim to be MVRA approved then contact the MVRA and inform them you have an issue. If they do not have this then it will require a second opinion from an appointed MVRA repair centre. Then it might require Trading Standards to get involved if you have paid them to do the work then the contract is between you and the garage. If it has gone through vehicle insurance then it is between the garage and the insurance company by law.