Rob wrote:

This morning I noticed a fairly significant bulge on the top surface of
my hot water cylinder

When you turn the hot tap on, does the pressure seem to be higher
than usual? There does need to be an expansion vent, which in my
country (NZ) is a pipe going straight up and is open at the top,
or else it's a small horizontal pipe going outside with an
expansion valve which may have a flushing lever on it to clear
foreign matter from the valve seat.