My Laguna Mk1 1998 1.8 8v RT uses it's water fairly quickly-like from maximum to minimum mark on header tank in 500miles or so. I can't see any leaks anywhere, the only thing I notice is the heater doesn't kick out loads of heat implying heater matrix may be duff so this is possibly where it's going. Another thing that bugs me is what the hell does this stuff in the header tank mean?

For the record the oil seems uncontaminated and the motor runs sweet (well about as sweet as it ever would[:p]) and I have drained, flushed and refilled the cooling system with the correct coolant mixture. Also, the coolant I replaced looked lovely and green-just fine in fact but I replaced it anyway.
Thanks