Once you have phoned for an emergency callout, you will have some time before your boiler repair technician arrives. The first thing you need to do is turn off all the water in your home, especially when the issue is a leak. This will likely cut off the supply of water that is leaking, minimising any water damage to your home until your technician fixes the problem more permanently. You can do this by turning off the stopcock in your house. You should already know where your stopcock is. If you do not, then check under sinks and other places with running water in your house, as this is usually where they are kept.