Property vacant for 2 months with no communication

My rental property has been managed by Ultimate Letting. Previous tenant moved out end Nov 2015. I agreed that agency should source a new tenant. Since then property has been vacant for 2 months with no communication from them. I then advertised the property myself. I passed potential tenants over to Ultimate Letting to review as i did not have the capacity. Ultimate Letting confirmed they were busy with reviews etc but after 2 months no comms received from them. I also received a number of calls from other agencies who wanted to take over the management of the rentals. After this I sent a notification to Ultimate Letting that I will be using another agency. When I call their office no one seems to know who is in possession of the keys. I am very suspicious of this behaviour and will now go and check if there is no one living at the premises without my knowledge however I am out of town and cannot act speedily.