Boiler repair

Our boiler loses water pressure when the underfloor heating is on. Pressure rises to about 3 bar, then drops to zero, and the boiler shuts down. Radiators and hot water work fine with the underfloor heating off. This looks like a common fault, so we expect a straightforward quote. I think the issues may be as followed hence the Work required: Locate and repair the leak in the underfloor circuit, check or replace the expansion vessel if needed, then re-pressurise and test the system.