As above, on the my Golf Mk5 1.9TDI, spotted what may be a leak on the coolant expansion tank. As you look at the tank from the front of the car, on the tank in the 10 o'clock position is a raised squared-edged section on the upper half of the tank. When I removed the explansion tank, just after switching off the engine, while unscrewing the cap, bubbles/collant were forced out on between off the seam of the raised section (where it meets the rounded tank) - I'm assuming this should NOT leak?!

Could this also explain why during running, the coolant level drops alot, and then when the cap is undone, pressure releases and the level raises back to normal (but there's also ALOT of air bubbles in the coolant).

Water pump is working, as well, and the car doesn't appear to be loosing coolant.