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Hot water cylinder assessment

I bought a house with a "mains pressure" hot water cylinder but I seem to have low pressure. I know there is a head tank in the roof but am not sure if it is in use. Most of the plumbing is behind the walls so I'm not sure exactly what is going on. I think the previous owners may have bought a mains pressure cylinder (years before they decided to sell) but kept the original plumbing. Is there a reason they would of not properly installed the new cylinder? Could I have mains pressure if it's properly plumbed? I'm not looking to replace the cylinder.