Golf Mk5 1.9TDI coolant looss

[VAGMAN2]

My wife has got a Mk5 Golf 1.9TDI (engine code BKC, and has done 130K), which has required the coolant to be topped up on/off for the last 12months. Previously when the expansion tank cap was opened, there was a big whoosh of air and the coolant level would surge up to max. This surge of gas has since stopped, but every 2nd week or so, the coolant light still comes on, requiring a small top-up of coolant. Got to get this fixed as it's been going on too long, and booked the car in for a pressure test on the coolant system, next week. What I have noticed in the past week is a small amount of brown scum around the neck of the expansion tank - so assuming this has been deposited by vapour?

I am a bit stumped on what the issue could be! There's no oil globules or film of oil in the expansion tank. In the past 12months I've done 2 oil changes and no sign at all of coolant in the oil (or on the dipstick or cap). There's no signs of coolant residue around the rad/engine/engine tray. Engine temp is also normal ie rises to 90degC and stays there. There's also no smoke coming from the exhaust. I've also pulled off the top feed pipe to the expansion tank, to check the water pump is working (and there is good flow) So I'm thinking (and hoping) it's not the head gasket. Suggestions have been made that it could be the oil cooler, or the EGR cooler (the BKC does have a water-cooled EGR as far as I can?).

Hopefully the pressure test will reveal more..........but in the meantime all suggestions/help very welcome.