There is a section on the naturalisation online application (AN) regarding spouse’s where it asks for the spouses Naturalisation certificate no. Unfortunately, my husband has lost this document( not made any copies too

Has anyone got any other idea? As this is the penultimate stage to what seems a never ending application process, I wouldn’t mind spending the extra £250, equally if there is a way around it , I would embrace this thankfully .
I look forward to hearing from the Mods( seem to find excellent support through this website more than any immigration services/even UKVI themselves).
Thanks
Nah