Mike,
If its the radiator Top Hose your replacing (Ive just done mine!) then its probably best to remove the Black Plug on top of the Thermostat housing (I think thats what it is!) anyway its easy to spot. You can then top up thru this, as well as thru the Resorvoir, this should then displace the Air in the Hose.
Word of Warning, make sure you use a close fitting spanner on the Plastic Plug, Its easy to round it off !
Cheers, Jubby