Simple things first. You sure you aren't losing coolant from a leak somewhere? If not could be the fan. Is it a mechanical fan belt driven off the engine? If so is the fan belt tight enough. If it's an electric fan is it coming on whern the car starts to get warm? Other problems could include a faulty radiator or faulty water pump. Can you get under the bonnet and have a look?
I've had 4 Corsa's and in two of them the engine over-heated due to a faulty thermostat - most likely yours is faulty - I'm not sure the age of your car but on mine it located behind the timing belt and this will have to be removed to gain access to it - if your belt is due for replacement anyway you can kill two birds with the one stone - but get the overheating problem fixed regardless or you could end up with a head gasket failure.
Check that your electric cooling fan is working it would normaly come in above 90 on your temp gauge.if not the chance is that your thermo switch on the radiator is not working,it not dificult to replace and should not be to exspensive..
Similar problem on my punto. I ignored it for a while and I learnt the hard way. My head gasket went and landed me in a �300 repair bill. Sold the car after the repair now and make sure you have checked your oil too as for some reason when it over heats the engine oil seems to burn out too. Keep an eye on water and oil and would get repair done ASAP to avoid more damage.