Even if you can't prevent a leak occurring in the first place knowing how to stop it quickly prevents the situation turning into an emergency. The first step is knowledge.
Know where your internal stopcock is and make sure it operates. If it's hard to turn a little WD40 often helps. Never leave stopcocks fully open, they will 'cease up'; back them off half a turn after opening fully.
Locate your external stopcock and note that in old cottages it may be shared with adjacent properties. It's great if you are on a meter because an easily accessible and simple to turn stop valve with be next to it. Make sure you can actually see the stopcock or stop valve as holes have a tendency to fill with dirt and the time to be digging it out is when all is well.
Know if you have isolation vales on taps on sinks, toilets and similar. In many situations one will be fitted. This can be simply adjusted by turning the screw throuh 90 degrees allowing you to stop the water getting to the tap or toilet. These valves are shown here.
If there is no isolation valve fitted then you will need to isolate the water elsewhere. Where exactly depends on the water system in your house and if it is a hot tap or cold tap/toilet causing problem. If the problem is a hot tap then call 50plus, who will ask you some questions about your water and heating systems and advise accordingly. If it's a cold tap or toilet turn of the main stopcock in the house. If this is not possible and you are on a water meter turn the water off at the meter outside the house. If this is also impossible call for help!
If you have a loft tank and your house was not built in the last 20 years or so and the problem is not the kitchen cold tap (in which case turning off the main stop cock should have stopped the flow) then there is a strong possibility that the tap or toilet is fed from a header tank in the loft. In this case to get the water turned off to the tap or toilet the header tank will need to be emptied. The actions to take are these:<
It should now be possible to access the offending tap / toilet or at least sleep peacefully.