My girlfriends '07 1.6 petrol focus' engine light has come on while driving, I checked the oil then topped it up to no avail. The fan comes on and is pretty loud but there is no noticeable loss of performance when accelerating, is it safe for her to drive for the time being?

To be honest, the issue could be as small as a missing sensor, or it could be worse. You really need to get an error code from the car to start pointing in the right direction.

Tried self diagnosis on the car and found error code E199 which seems to relate to a faulty sensor unit, is this expensive/ time consuming to fix?

That code needs to be translated anyway as they aren't true error codes.

I think your looking at needing to clean the temperature sensors, I would personally clean the maf sensor, thermostat connector at least

The engine is running fine apart from the fan noise, would it still be okay for her to drive it for a few days?

Keep it to minimum required journeys only. If it's a blown valve or piston (unlikely but the point is) you don't know it, what it is, or how badly you can damage the car, it could be nothing but it could be a big problem.

Code E199 means 'Erratic Temperature Readings' which will either be due to a faulty temperature switch or a dodgy connector/wiring problems. Explains why the fans are on constantly as the ECU puts them on as a failsafe. Try removing the sensor plug, spray with WD40 or switch cleaner and reattach the plug. Always try the cheapest fix first!

Car is normally ok to drive with the light on but if it starts flashing you should stop the car asap.

Thanks for all the tips and advice guys, turns out it was a loose wire coming from the sensor, bit of solder and it's sorted.