so far my temp has been getting a little higher even though it's bloody cold in in Perth, and when ever i put water in about 90% of what's in the header tank is let out through the overflow pipe.

It is let out in one driving trip and when i open the cap i can see about 20mm of coolant on the bottom, is my cap fugged?

more than likely.. get a new 1 they are under $10

hows the thermostat?
if its over heating i'd lean towards that first.

im thinking cap. i stressed i just had my motor all rebuilt and bout a month later i parked and as i walked away i saw all my coolant spewing out so i try that one first