Expansion Bottle; what's it all about?

[gt_james]

Think I've solved it, the upper rad hose had a very slight leak on it, barely enough to let any amount of water out, but enough to prevent the cooling system building a pressure difference as it cools down and drawing coolant back in from the expansion bottle. After really tightening down the hose clamps, the level in the expansion bottle and top of the radiator seems consistent for the last few journeys, but will keep an eye on it...