The kind of keg/pressue barrel I am using is known as a 'king keg'. it is the first barrel you see at this URL: "http://home.btconnect.com/thefear/beer-and-wine/barrels.htm". I have an O2 gas cylinder which can be screwed onto the top of the lid. I was led to belive that this gas was only to be used to help push the beer up from the bottom of the barrel.
Just to confirm what you said: I should allow the beer to frement as normal, then "... you can add about 200g dextrose dissolved in water to the keg then siphon the beer into the keg insuring it gentle mixes. Seal the keg and it will then carbonate the same as in bottles."
I can only try all of this when I locate a heater. Many thanks for you feedback. I hope to aquire somekind of heater in the coming days and given that it should take about 1-3 weeks for the cider to frement I hope to let ye know in about 5 weeks time how your suggestions went.
Many thanks in advance.
Thomas.