Hi max, perhaps your thermostat isn't working. It allows hot coolant from the engine to the matrix when the engine is warm enough. An old trick was to take it out and heat it in a pan of water to see if it opens ok. Worth a try!
I had similar experience with my Meriva- the problem with that was that the thermostat was quite inaccessible . I was able to get an 'in-line' thermostat which just fitted the end of one of the large radiator hoses -and has worked quite happily for the last 6/7 years....
Not sure where the stat is on the Corsa....