Emission light

First thing I would do was check water / coolant level and top up if needed .If down , you need to find where leak is from as this may be why car temp gauge and warning light coming on .. Could be something simple like water pump leaking but may develop into something major if left .

Thanks for that. I checked both this morning and while both were slightly low (I did top them both up, I don't think it's that causing the problem. On the drive to work this morning I didn't sit in much traffic yet the temp guage went up to half way (which isn't usual but since I've had this problem, it is). On the 2 occasions when the emission light has come on, both times have been after journeys where the temperature went up, I arrived at my destination, switched engine off then not long after was back in the car, arrived, engine off etc. So it seems it was coming on after engine was heating up and if it didn't have chance to cool sufficiently before starting up again. Usually I drive to work and the car sits there all day until I go home so it has time to cool down. It may be a stupid question but is it easy to tell if the water pump is leaking? Could it be something to do with the temperature guage?