Edward W. Thompson wrote:
What to do? The cylinder should not be used and replaced. The cost
will be about £120 if you do it yourself. I just replaced a leaking
cylinder, the cost was £104 from Wickes. I also took the opportunity
to replace the immersion heater about £24 if I remember correctly. You
can use all the existing fittings on the cylinder. Time to drain
down, remove the cylinder and refit, about 4 hours provided there is
reasonable access..

Don't *forget* to notify the council's "building control" department,
and pay the relevant fee.