Rent increase form 4, but a problem!

We inherited a couple of tenants when my parents passed.
They have not had a rent increase since at least 2020, possibly a few years prior.
The issue is that we do not know the date of the last rent increase for para 3 of Form 4.
Can I just put “Unknown” which is the honest truth? O doubt either tenant will have kept records themselves, and we have found precious little whilest going through the paperwork.

I think thats all you can do.