Leaking heater core?

[irext]

My AU series one 6cyl has started regularly fogging up the windscreen when the demister is on the first two fan speeds with or without heat turned up. I think it's worse with heat wound up but not conclusive. If the fan speed is set to three or four it defogs o/k but the noise drives me nuts. Running the aircon fixes it also. No coolant loss is evident from the radiator reserve tank so if it is a leak it must be a pinhole. It never used to do this so something has changed but before I pull the dash and everything else out to get to the heater core I thought I would see if anyone else has had a similar experience. The car has done 240000 very reliable kms so far but I guess it's at an age where these things can start to fail. I am somewhat reluctant to simply send it somewhere to get fixed and have someone possibly do a rush job of refitting the dash and create all sorts of other problems so I'll almost certainly do it myself. (Ive been burnt before).