I have a 2ltr zetec with a rover 25 header tank and polo rad. can i mount the header tank lower than the engine but higher than the rad, the rad has a bleed pipe?

really should be the highest point in the system, so that you don't get air collecting in the highest point (your cylinder head!) coz that would be bad

Hi

That would be similar to the rover setup in the Caterhams. It didn't work. The header tank and air separation need to be at the highest point.

Cheers Matt

i get the feeling it would work when the engine's running, as the pump will keep water pressure up in the block forcing air out, but as soon as the engine stops, i think it will drain out and cause heat soak problems when the engine's cooling

As Matt says.
As a MINIMUM, the level of water in the bottle should be no lower than the highest point of the water in the engine. This won't be the highest point on the engine but usually the height at which any coolant bleed pipe comes off the head/manifold/thermosat/etc.
You might get away with less but cooling has been known to be marginal on sevens so better not to risk it.

adrian