The obvious thought is radiator but just wondering if this rings any bells with anyone?
Does the water pump have a vent for when water is getting past the first seal/bearing like others I have come across? I've had a squeel or two.

Its a 300 tdi with air con.

Coolant level going down in the header tank slowly but the water on the X member doesn't stink of antifreeze so not sure if it is a red herring!

Cheers.

Does the header tank smell of anti-freeze? If it's leaking has it been topped up with just water and therefore just water leaking out?

Not had it long but have flushed the coolant system and stuck new anti-freeze solution in. No sign of over heating yet and the heater is toasty.