Disco TD5 coolant question

[chizy]

Hi all, any advice appreciated because I'm scratching my head with this one!!!
Have just replaced fuel cooler on my 2000 TD5 manual and seem to be loosing a bit of water.
At first thought maybe I hadn't put enough coolant back in to compensate for the fuel cooler remove/replace. Topped up but still lost a bit over the next couple of days. No visible signs of leak underneath. Just been serviced so oil is clean and no signs of water in filler or dipstick. Have replaced expansion tank cap yesterday as the old one was a bit rattly. No obvious signs of contamination in the expansion tank. Heater working as it should be. Hose pipes not like concrete.
Today driving to work (4.5 mile trip), temp showing just above blue square then crept up to normal while at red traffic light. Just pulling into carpark a couple of minutes later and gauge had dropped back to above blue square and then up to normal as I was parking up.
So there we have it, what's occurring?
Head gasket.............? 

[Range Rover Blues]

I'm not totally familiar with the TD5 cooling but from experience with our TDi, if the head gasket is going you'll see evidence of bubbling etc in the expansion tank, also I had a recurring airlock in the thermostat.  The unsettled temp gauge suggests a circulation problem

If you are loosing water you can't find check that your carpets are dry, could be a leaky matrix.

Someone who knows better should be along soon

[chizy]

Hey Blues, thanks for the response.
Carpets dry, pressure test all good etc.
Going to check for airlock next.........
Still reckon head gasket