It's sometimes easier from under the car, and you might need the proper tool to open the spring clamp.
http://www.abbeypowertools.co.uk/car-repair-tools/cooling-system-tools/showitem-12542-89793.aspx
(similar tools cheaper elsewhere, that's just the first link I found).
Once you get the clamp off use a funnel and hose to pour boiling water on the hose, it'll come off a lot easier.
Having said that, unless I need to get the radiator out I dont bother with the bottom hose.
I just remove the top hose, slip a length of thin hose down inside the radiator and siphon the coolant out. The way the Vectra rad is made (the bottom hose is halfway up the rad) you actually get more coolant out this way.
fred, January 2011
