i am loosing water,back pressure through header tank,vauxhall vectra 2 ltr w reg,123,000 miles,no signs of head gasket wear,temp stays normal after 50 miles,no oil or water in each other,but car has been standing for 6 mths.

brian leighton, October 2009

Get a sniff test done. This tests for exhaust gases in the coolant bottle. I suspect the head gasket has failed between one of the cylinders and a waterway.
If you are mechanically competent you can buy your own kit, with enough test fluid for dozens of tests, for the cost of one test at the garage!

http://www.tools-n-all.co.uk/Universal-BT500-Block-Tester-Kit/?gclid=CN3uv97J0J0CFdBb4wodlA1OsA

Google "block tester" for more.

vaux, October 2009