Not sure if this document is up to date, but it states that tax must be paid on rent receivable as per the lease, not the actual rent paid. So if you receive less than stated on the lease, you write it off each year as a bad debt. Would you need to prove it's not recoverable? As Gordan said, an odd email about the unpaid rent would show you tried to recover it.
"Members should be aware rental income is taxable on a receivable basis i.e what
you are entitled to receive as per the lease rather than what was actually received.
Any unpaid rent, if not recoverable, can subsequently be written off as bad debt and
a refund can be claimed for any tax paid on it."