my ford focus has a warning light which has just started coming on its a cog with and exclamation mark in it. it comes on when you start the engine, but if you then turn engine off and restart it it goes out. does anyone know what this is for or how to rectify it please

A little bit of information regarding your car please , diesel / petrol , miles etc

A cog with exclamtion mark i think is powertrain , i would be inclinded to take it to the dealer and get it checked out

Yep powertrain fault.
You may find that you will end up getting the limp mode coming up.
It would have logged a fault code on your system which will point in the right direction.
It could be Mass Airflow Sensor, injectors, and if a diesel the glowplugs and/or glowplug relay.
Best diagnosed by a reputable garage.
If the light is steady then it's not going to go bang suddenly, but if it starts to flash then i recommend you stop as soon as possible/safe to do so and get it looked at immediately.