1. As a landlord it is your responsibility to ensure your tenants have heating and hot water. However, they cannot expect instant attention.
2. Have a look at the installation instructions for your boiler to see how the system pressure is topped up. If you don't have them they are available on-line.
3. From memory it requires two grey plastic handles under the boiler to be turned through 90 degrees. Water will enter the system and re-pressurise it. When it reaches around 1.3 bar, close both valves.
4. You could:
4a. Explain to the tenants how to do it. It is not unreasonable to expect them to be able to undertake such a task.
4b. If you are comfortable with the method, go round and do it.
4c. Get a heating engineer to do it, perhaps the people / person who serviced it.
5. Get your tenants to keep an eye on the pressure. If it drops soon after being re-pressurised, it suggest a fault with the heating system or the boiler.