If your loosing coolant and not being able to find the leak my advice is to top it up, have it running hot till the coolant fans cut in. Then turn it off and look in the morning under the car. Hopefully you'll see where the leak has come from when all the pipes and components have cooled (easiest way to be honest, without a coolant pressure tester)

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should i fill it up start it and turn the heaters on full whack on warm ?

ill have to check that tomorrow and see i did put a new rad hose on so that could be leaking. no not really the heaters just take a while to get hot prob because the weathers got stupidly cold

Have you checked for leaks with the engine running? Generally leaks aren't obvious with the engine off. When it warms up the system pressurizes and the pressure means the coolant can escape so if you haven't tried that, that's the first thing to try, other than that you can check for signs of leakage inside the car it could be the heater matrix or one of the pipes to it causing problems. The other obvious thing is potentially the headgasket so check for mayo on the underside of the oil cap, have you been having any temperature issues, overheating, heaters blowing cold etc.? White smoke out of the exhaust?

my header tank is really low and i topped it up a few weeks ago and i cant seem to find any holes anywhere or drips :s so wheres it going ??