Thanks for your question. Please kindly RATE my answer when you are satisfied
Your claim is as you know against the tenant from who you originally let the property. You cannot distrain his furniture without a court order. Consider issuing proceedings against him in the county court and following judgement obtain a warrant to seize his furniture to sell to offset against the debt.
The simplest way to issue proceedings is by the money claim online service.
Does the above answer all your questions or is there anything I can clarify or help with any further?