What size reservoir does the one you linked have? I imagine if it's a small reservoir the hot water could expand back in to the mains without the need of a vessel. But then have to considered if a single check valve is needed. Depends on how close the nearest cold water draw off (tap) is
Thanks. It's just then if the unit does comply with UK plumbing and electrical regs. Electricians would be wiring it up so they would take a dedicated supply from the main board.