Hi SallyB,
I agree that you have to declare the rental income and
Use of trading income rules: splitting receipts and expenses between years - PIM1101 confirms this.
The only other thing I can find in the Property Income Manual is
Deductions: main types of expense: bad and doubtful debts-PIM2072 about bad debts and them becoming irrecoverable.
However that guidance does state this may only be allowable after you have taken all reasonable steps to recover the debt. I would say in this case you must pursue criminal proceedings against the estate agent to recover the money and it would only be after you have exhausted those avenues and can provide evidence of the action he you have taken that we may consider allowing relief for a bad debt if you have not been able to recover the money. So ultimately you have to include this rental income initially.
Thank you.