You can apply to court for a civil injunction , enforcing the contract, to gain access to the relevant project.
Before you approach the court , you need to send a cease and desist letter to this client to warn them of your intentions unless they stop (and give them 7 days to stop their conduct) giving you access .
That letter may well be enough to stop the conduct - as an injunction is a serious legal remedy.
Assuming they continue, you will need to complete the application for the injunction.
Kindly click below for the form you need for filling injunction-
The application must state what order the applicant is seeking, the reasons why the applicant is seeking the order, supported by evidence. This will usually be in the form of a witness statement or affidavit including all material facts of which the Court should be made aware, and attaching relevant documents.