Any question answered...

If you had a contract, I'm sure you did, with the landlord have a look at the small print they might mention such a scenario and would probably charge you an admin fee for return of the cash. You never know.
There is also the fact that you have paid for something they have not provided so if it gets to it threaten them with legal action regarding unjust enrichment. Two people I know had a very similar thing and it worked then for car insurance and a sofa.

This sounds like sensible advice. Thanks.