Agree, it's a 4 gallon feed and expansion tank for the heating. If it
has distorted, maybe it's had very hot water in it when the heating was
running; this would indicate a fault. It should always be cold.

You probably need to replace the washer in the float valve/ballcock. It
should have about 1" or 2" of water covering the outlet when the
heating system is cold, leaving about 3 gallons of airspace to
accomodate the expansion of the water when the system is hot. They are
very frequently set up incorrectly, with 3 or 4 gallons of water when
cold. The water will then overflow when the system heats up, and will
take in cold water when it cools down. This will ruin the heating
system in a few years.