The answer
Why might the existing cylinder be unsuitable?
You may need a different hot-water cylinder, but not simply because the system is a heat pump. The decision depends on the existing cylinder's heat-exchanger coil, capacity, recovery time and how your household uses hot water.
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Why a cylinder can be the bottleneck
A heat pump typically supplies water at a lower temperature than a boiler. A small cylinder coil may transfer heat too slowly, especially when several showers are used close together. The survey should establish daily demand, peak draw-off and the available space before selecting capacity. An oversized cylinder also wastes room and can increase standing losses.
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Include the controls and hygiene plan
The proposal should explain how hot water is prioritised, what temperature is normally stored and how the system's required hygiene cycle works. Ask whether an immersion heater is used and how it is controlled. Check the physical route to bring a new cylinder in, not just whether the cupboard dimensions look large enough.
Where this guidance comes from
Check the original guidance.
These primary sources support the technical or scheme points above. Rules can change; follow the current source before making a commitment.
Apply this to your property
Check your hot-water setup before choosing a heat pump.
A free survey can assess the existing cylinder, household demand and practical replacement space as part of the full heating design.