I need some advice on the best way to drain down my CH system to add
an additional radiator. I'm in a first floor flat and the pipes run
under the wooden floor. There is a drain cock on one of the radiators
but correct me if I'm wrong this will only drain to the level of the
drain cock? All the pipes under the floor will still be full of water.

My thoughts were to buy one of those self piercing taps that you can
use for washing machines etc which you can fix without having to empty
the pipe of water. I could fix this under the floor and then attach a
length of hose to this. The hose will of course travel uphill for a
short length as it passes up to floor level from under the floor.
Could I syphon the remaining water out ?

Any other ideas bearing in mind my requirement of not wanting to make
a mess of downstairs ceiling!