So I have soaked and scrubed my boards today with the large one not fitting easily into the bath. I won't soak them over night as family members need the shower which is over the bath. No point risking them being heat damaged.
Will follow this advise over the next year. Place wet towels over the boards for a night. My boards maybe to used and old but I am sure if you have just purchased one, you will be able to really test this.