Hello Paul,
Here is the
official MG Rover method for reference but it sounds as if you have already been following it.
When undoing the expansion tank cap on a cold engine a slight hiss, akin to opening a beer, is nothing to worry about.
If I were you Paul I would drain your cooling system fully, i.e. using the LH bank cylinder block drain plug as well as removing the radiator bottom hose and start from scratch. If you can, rig something to support your expansion tank leaving both hands free. Note from the official method that
as soon as coolant starts flowing steadily from the bleeding orifice, the screw should be refitted. This shouldn't occur until you've poured quite a lot of coolant into the system. If it happens immediately, something's amiss.
By the way, the bleed screw is in the
bottom radiator hose on its way to the thermostat.
Any comments?
Simon