It's worth a shot, thanks pal. Will update the thread after I have done the work...
Not to suggest you have been given wrong advice, but experience of draining my wallet has taught me to diagnose a problem before throwing parts at it and hoping it was the problem. Coolant loss is one of them problems thats a swine to diagnose as it gets pumped everywhere and could be lost at any point, some cheap and easy to fix while others are expensive and labour intensive. Personally I would check everything, starting with the more serious first:
Is there water in the oil?
Is there white steam coming out of the exhaust on start up after its been sitting overnight?
Is the cooling system becoming pressurised? When that happens it can chuck the fluid out of the expansion tank. If possible, run a chemical headgasket tester on the expansion tank.
Re-check everything for external leakage. Remember that it could be leaking from hidden places like the heater matrix behind the dash, or from the radiator which leaks on to the plastic tray under the bumper and is hot so will evapourate. It's more likely to spot signs of evapouration and cleaning than it is to spot the leak.
If the answer to any of the first three checks is yes, then it usually indicates failure of the thermostat housing, head gasket or the engine casing. If they are all negative and the only thing left is all the external leaking points, you can take your time in trying to find the leak as long as it's always kept topped up.