Ask an Property Solicitor. Get an Answer ASAP.
In theory a diplomat is immune from the civil and criminal laws of the the state he works in. That could mean for example that if he trashed the property he wouldn't get prosecuted and you couldn't sue him for the damage, not indeed the rent if he decided not to pay. Those are the risks. As against that in practice it is very unlikely that a diplomat from a foreign country would wish his country's reputation to be damaged by such behaviour. Whilst it is a decision for you, I would make a judgment on reputation as opposed to what the law actually says here. I hope that helps. Please leave a positive rating so that I am credited for my time.