Can I Enter if Tenants Have Surrendered Early?

We are Landlords and have agreed to let the tenants surrender the tenancy agreement after signing it in December. The tenants left our property at the end of January. We believe our property is now empty.  
 
However, having sent emails to the managing agents for paperwork to this effect, we are getting no response. We are in a position where we cannot enter our property if the tenants have not vacated. However, if our property is empty, this affects our insurance policy, and we also need to check the heating and other properties to ensure everything is in order.  
 
Can you please advise what the managing agents should do, i.e., paperwork to end the tenancy, keys handed in, etc.?