It is an expansion vessel and part of your boiler, as the system heats up the vessel takes take`s the expansion pressure, can`t say why it looks like it does but everything only last so long, good news is it won`t cost an arm and a leg to replace it.
looks like an expansion vessel attached to an unvented hot water cylinder,if it is ask your service agent who does your periodic unvented cylinder servicing for advice.
What cylinder is it attached to? If it came with that you might be lucky and still covered by warranty! Worth a try - send photo of the EV to the UV cylinder supplier.
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