Trying to remove radiator - water coming out every time unfasten the radiator

In your attic their would of been a small rectangular tank, we call the F&E tank (feed & expansion) this provides make-up water for your central heating system and also a means of temporarily storing heating system water due to expansion caused by over heating or blockage. This tank will have been de-commissioned before your back boiler was removed or else you will have had water constantly filling up your heating system and discharging/leaking. I would assume that the loss of water is due to your heating system not being drained properly, not being privy to your system layout locate all ground floor drain cocks and place a hose over the drain cock and try and remove the water by this means. How ever, if you hear the cold water storage tank filling up as you carry out the aforementioned draining procedure report back and I will advise you further! Update: You have turned off a gate valve, which will isolate your hot water cylinder from the main cold water feed from the storage tank in your loft? The other tank you mentioned is the smaller heating F & E tank, the cold water feed to this should of been isolated properly, certainly not by just tying the ball cock up with string or such like! This still brings me back to my first post, when you try and drain down your heating system can you hear your large cold water storage tank filling up as you do this?

see your loft if the cold water tank is disconnected!