Turn the water off
Find your internal stopcock, usually under the kitchen sink, sometimes in a downstairs loo or utility, and turn it clockwise to shut the supply. That one step stops most emergencies getting worse. If you can't find it or it won't turn, don't force it, just ring us.
Kill the power if water's near it
If water is anywhere near sockets, the consumer unit or an appliance, switch the electricity off at the fuse board first. Water and electrics are the part you really don't want to get wrong.
Drain it down and protect the room
Open the cold taps to run the system dry, flush the loos, and catch what you can in buckets and towels. Move anything valuable clear. Then call us and we'll talk you through the rest while we're on the way.
The ones that genuinely can't wait
Some things will happily wait until morning. A burst or badly leaking pipe, water anywhere near your electrics, a smell of gas, or no heating and hot water in the depths of winter are not among them. If you've got any of those, get the water or gas off and ring straight away. A dripping tap or a slow-filling cistern is a next-day job, so you're not paying emergency rates for something that can sit.