Corsa C coolant leak ?

Compression test? Oil pressure test?

vauxhall's do suffer from the "mayonnaise" problem in the oil filler cap. you say they did a coolant pressure test or did you mean a cylinder head combustion leak test ? you can buy the kit on ebay

,bit of an over kill though if your only going to use it the once. but the garage should have the kit to do this.try putting a piece of cardboard underneath the car after it has been run & re-check it in the morning, that way if you get any water patches you will have an idea where its come from, it may well be the water pump leaking after the engine has cooled down.

You often get mayonnaise at the oil filler especially with short drives. It doesn't get hot enough to boil off the moisture. Providing the oil is fine on the dipstick it's ok.

What's the oil level like? It's the first thing to check if the oil pressure light is flashing, especially on cornering.

Look for water leaks while the engine is running. Sometimes they can leak at a join when cold or when hot. Also the pressure cap on the filler bottle can fail and allow water vapour to escape. You are using the correct mix of anti-freeze and water to top it up I hope. If the anti-freeze concentration gets low it can cause all sorts of problems since it contains anti-corrosion additives as well.

How much coolant is it loosing by the way?

Ignoring the other symptoms for now, have you checked the heater matrix for leaks? Just changed one for a relative and was told by the parts place they are a commonly sold item for the Corsa C...

I wonder where the coolant is leaking from. Could be worth checking the likely suspects, I'm no expert I might add. I'd start looking at the radiator, hoses, thermostat housing, coolant temp sensor, also if the seals on the coolant cap are in tact as when pressure builds up it can leak out of that if the seals or lid are past it. (not vehicle specific) just what I usually look at.

Radiator on mine had a small weep that took me ages to find.

IIRC it was on the bracket at the top FNS it only weeped when hot (and probably only then when vibrated.

Changing Rad was a peice of cake, just check the cast brackets before you start. They are cheap as chips anyway.