Heath Robinson is my friend again, but after about a week the tank has shrunk enough so that it almost fits again!
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It consists of a small bucket with a tight lid and a hole cut in the bottom, the filler hose from an old petrol can (cut down, big end taped up to help with sealing and wedged into the tank filler hole, smaller end stuffed through the hole in the bottom of the bucket) and a 69p disposable dehumidifier in the bucket.
The thinking is that the dehumidifier removes any moisture from the air in the bucket, creating a drier area that the moist air in the tank then tries to balance by removing the moisture from the tank. Living so close to the sea means that the normal air is pretty damp all the time anyway, and mist and fog is quite common so just leaving it open might have made it worse!