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Hot water cylinder overflow with possibly no trap?

We’re selling our home and the purchasers have come back with the below issue. The builder says the smell in the hot water cupboard comes from the sewage pit. The overflow for the cylinder leads directly into the sewage system. It either does not have a proper trap or has no trap at all. John le Prou the builder says it would have to be investigated by a plumber. Photo of hot water cylinder overflow attached below. Looking for get a plumbers opinion/fix options if needed. Given that it is a live contract we’d love this sorted This week if at all possible - Thanks