2001 Passat TDi130 Coolant Loss AWX Engine

[TDi130Passat]

Hi, I've read similar threads but here is the deal with mine. Bought it about 4 weeks ago 160K and it drives well I've done about 700 miles so far. I noticed whilst doing a service that the coolant tank looked stained but when I extracted some coolant and put it on a white surface it looked pink, the coolant looked orange in the tank. I removed and cleaned the tank as best I could and refitted it, refilled it with G12 50/50 with water mix.

The new oil is black and no trace of mayo and I haven't lost any oil in the 700 miles but I am losing coolant at a rate of half a litre in the last couple of weeks (covered approx 300 miles). I've since bought another tank and given it a good soak and clean but before I fit it I was wondering if I should do a coolant flush first to see if I can get rid of the orange looking stuff in case this was a mix of the wrong coolant in the past?

Second thought is it could be the radiator but I haven't checked that yet, apparently they can go N/S bottom corner and I haven't put the under trays back on yet so can't really tell if it's collecting in that region.

Third thought it could be losing it through the expansion outlet so I'm going to do the bottle test this week and see.

There is a reasonable amount of pressure when you release the top when it's cooled down after a run but not excessive.

I really hope it's not the head gasket but how should I carry out the checks above and any idea on likely causes?

Thanks in advance,