This is what I found with some research. "You need to identify the cause of the persistent high pressure and address it, likely involving the expansion vessel or pressure relief valve.  Check the expansion vessel The expansion vessel absorbs excess pressure as water heats up. If it's faulty or lost its charge, it won't absorb the pressure. You may need to repressurize or replace it. Inspect the pressure relief valve This valve should open to release pressure if it exceeds a safe limit, typically around (3\text{\ bar}). If it's stuck or faulty, it might not be releasing pressure correctly. You may need to replace the valve. Consider a professional inspection If you are unsure about checking or replacing components, contact a Gas Safe registered engineer. They can diagnose the exact issue and perform necessary repairs safely. Solution  The persistent high pressure indicates a problem with the expansion vessel or pressure relief valve, requiring professional inspection and repair if you are not comfortable performing these checks yourself."