At the weekend, we noticed a damp patch on a downstairs ceiling, right under the hot water cylinder (in a cupboard upstairs). At that point we went off on a couple of blind alleys, but then noticed that the cylinder has a bulge in it where the dome meets the vertical bit. The insulation has cracked of course.
Getting it fixed (not a job I have time to DIY right now)!
My question: is this a common failure mode?