Chances are that you'll need a new cylinder. Mine sprang a leak a few years ago. It was weeping from the seam around the top. I managed to seal it for a while using a product called Chemical Metal, made by "Plastic Padding". I say for a while; it started leaking again after between 9 and 12 months so I changed the cylinder.
To seal it, I turned off the mains water, ran some hot water out of the system to get the level in the cylinder below the leak, dried it off then applied the product. Let it dry for an hour & turned the water back on.
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