Ford Focus steering clunk /knock when turning left

Hi, we have a problem with our 2004 Focus 2.0 Estate, owned since new regularly serviced each year, developed a clunking /.knocking noise when turning left, what happens is you start to turn left and you get some resistance like it's sticking, then it clears but you get the clunk/knock noise, it always been when started from cold and it usually went away after car warmed up. Have been to several mechanics who have all been baffled stating that it could be anything from : steering rack, steering pump, top mounts etc etc, as the car has done early 90000 miles and is only worth approx £600 we didn't want to just start changing things at random, on advise of another mechanic we changed the top mounts as it was the cheapest thing to start with, this did improve the problem briefly but it has returned, it is getting worse where it is happening even when the car warms up, have been to a Ford main dealer where I used to work and the service manager says he believes it's the steering rack, if we are to replace this we are looking at a cost of over £500 which is fine if it resolves the problem but seeing as we can't get anyone to give a definitive answer.... Can anyone help???!!