Hi. It's a 2005 Mk1 Focus 1.6l 3 door (62k miles). The leak if from the rusty looking casing at front passenger side near wheel bar. Mechanic thinks it's Power Steering leak but level doesn't ever go down?

It's a light oil and is slightly coloured, leaving stain on drive behind front driver's tyre. Got photo but not easy to add from dropbox.

Anyone else got this problem or know what it is please?

You can directly add photos to posts if it's on a phone, click 'choose files' link at the bottom of the text box.

I'm a bit confused by the location, you say it's on the passenger side but leaving a mark behind the drivers side wheel?

Power steering fluid is quite thick, and usually dark red so you should be able to distinguish that.

Brake fluid is a very light colour and feels oily, and that could drip right by a wheel.  Also shock absorbers can leak oil near wheels.  The only other light, slightly coloured oil would be gearbox oil but that wouldn't be close to either wheel, more central under the car.