As above, no it's not. The wiring adapters are really quite simple. Unplug connector/connectors from old head unit, plug into adapter, plug adapter into wiring harness that comes with new head unit then plug that into new head unit.
Only a little more complicated if you have steering wheel controls too. Even then it's basically the same, just there'll be a little box that connects to the adapter harness then you need a patch lead to go from this box to the new head unit which varies depending on manufacturer.
Do you have steering controls? Adapter harness for no steering controls is probably going to only around a tenner. With steering controls probably around £20-£30.
Oh, I believe for the Corsa you will need an aerial adapter to plug your aerial into the new head unit too. Usually only 3 quid or so. Incartec site linked to above seems to be a good bet for filtering down to what you'll need.