Where's the stopcock?

A drip-drip-drip sound is not a good start to the day. A drip-drip that soon produces a puddle -

A puddle right in the middle of the floor? The pipe to the radiator upstairs must have blown, that's the only thing that's near that area...

My son wasn't happy to be wakened so early, nor was he at all pleased to see the puddle. The first thing he did was check the pressure on the boiler (we had a similar problem before, which dripped into the flat downstairs, eek) - it was fine. Find the stopcock and turn off the water till we figure out what's going on. Drill a hole in the ceiling so all the water can drip from the same place and be caught in a bowl. Hurry back upstairs, contemplating having to move furniture and tear up the carpet. And floorboards.

Then he remembered the water tank under the eaves, and checked that - and indeed that was the source of the problem, with the water travelling to the middle of the ceiling because that's the lowest point.

Turn off the water supply to the tank, hurry downstairs to speak to the neighbour whose tank it is. But what's this - he's had a condenser boiler fitted and no longer needs that tank, his water comes from the mains. Well, if he runs out of water, further action will be needed. Wait and see...

I don't know what I'd have done if I was on my own. Got the bucket to catch the drips, and turned off the water - that's as far as my thinking gets. "You'd have phoned me," said the son. Yes, I would have.

In a previous flooding incident we learned the hard way where our stopcock is. Or rather, we had it moved to somewhere easily accessible.

Do you know where, should need arise, to turn off the water?