I assume you mean the one that delivers to the hw cylinder?
Means getting in the loft and inspecting - proceed with caution if it’s not boarded out.
If it’s constantly running into it, then if there’s an isolation valve, isolate and check the float valve over and see if it stops, if it’s does then there’s an issue with the valve: seating damaged, seating blocked with debris, arm adjusted to incorrect level, not shutting off on the washer correctly, water in the float, damage to the float, check it’s not the vent pipe - just a few ideas to check.