Losing a lot of water. Where to look?

[Horney]

Hi folks,

Got a small problem with water useage in the old wagon. I'm getting through a pint and a half every 100 ish miles. Clearly something is not right somewhere. Are there any common weak spots in the coolant system? So far I have checked the following:

* Heater matrix replaced by last owner (No fogging of windows so think this is OK)
* No Mayo under oil cap and runs fine so I don't think it's the HG
* No visible leeks from expansion tank, pipework or radiator.

I'm a bit stumped really. Seems to use less on long runs but more if lots of standing in traffic. I see no steam from under the bonnet and I've left it idling for ages and had a good look round and can't see any coolant loss anywhere!

Nick