Rob wrote:
Hi,

Is it a cylinder with foamed on polyurethane insulation, or a separate
insulation jacket? If the former, is the bulge in the copper cylinder,
or just in the insulation?

I replaced an immersion heater on a hot cylinder once and a small
amount of water spilt from the immersion heater boss. There was an
alarming crackling sound (it sounded like a bonfire) and the insulation
around the bottom of the cylinder started bulging and splitting. It
appeared to be that the water was evaporating and blowing the
insulation off, or that it was some chemical reaction between the water
and the polyurethane insulation. The cylinder wasn't hot enough to boil
the water.

If it's just the insulation bulging, it might be just a leak from the
immersion heater causing the insulation to separate from the cylinder.
remove insulation and check.