Now would be a good time to find it - rather than at 3 am on a winter's night when you've got a burst pipe and no way of shutting the water off. Usually on the property boundary/pavement.
A head off a stopcock MAY fit your existing stopcock http://www.screwfix.com/p/15mm-stop-cock/31490 - but it may well be seized if it hasn't turned for quite some time. You might have to enlarge the cut-out sections of the cabinet to allow you to turn the handle. Next job, find and soundly beat the muppet who took the handle off the internal stopcock!
Why not get a socket that fits the valve spindle & use a ratchet to turn it off/on?? Of course if you plan to live in the gaff for some time, cut the kitchen base unit & fit the correct handle. As others have said; know that your external stopcock in the street is working too.