how can i fix F5 fault?

Hi, we rent out a property and have a letting agent to manage it for us. They have contacted to say that the boiler has a fault F5 and the engineer has said it is because it has sludge in the system and will need to be flushed. We are concerned that we are being conned, especially as reading about pressing the reset button. Are they are suggestions on what we can do, as being a landlord we have to make sure that the tenant has heating and water.