This will do the job or even a tap connector.You can screw it on and join your pipework to it.
http://www.toolstation.com/shop/Plumbing/d20/Compression/sd2697/Compression+Coupler+Female+Chrome+Plated/p93355
Sorry...just read all your post and that might not fit.
You could dig out the cast iron and then connect to the clay (or cast) using this
http://www.toolstation.com/shop/Roofing+%26+Drainage/d250/Soil+%26+Vent/sd2849/Flexible+Adaptor+Clay+to+PVC/p54240
Most gate valves on the cold feed are higher than the cylinder so turn off gate valve,undo the lower nut and push a hose pipe down the pipe to the bottom and syphon...simple.
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Drain the system and cylinder depending on whether it is a direct or indirect cylinder and cap pipes or plug the cylinder with 1" plugs.
http://www.screwfix.com/p/brass-flanged-plug-1-pack-of-2/74007?kpid=74007&cm_mmc=Google-_-Product Listing Ads-_-Sales