There are a number of reasons a floor squeaks. Ironcially, one of the most common is the fact that original floor was put down with nails, which have worked loose and when one walks on the floor, the flooring moves up and down on the nail, producing the noise.
First, if the subfloor can be reached from underneath, such as from a basement, there are devices like this:
http://www.installers...vW28K0CFYXd4Aodm3F8sA
That I can attest work very well, are reasonably priced and are easily installed.
Barring that, if you'll locate the floor joists (the heavy duty, usually 2 inch wide supports for the floor structure) and if the house is reasonably new... and if building codes in the U.K. are similar to ours in the U.S., they'll be 16 inches on center apart. Meaning, when you locate one, the next would be easy to find, just measure. Once located, drive at least a 2 inch 'deck' screw through the squeaking area into the joist. Drive it far enough to somwhat 'sink' the screw so its head is below the level of the flooring. Continue until the squeak is amended...