New sink causing awful smell

[Slugbabydotcom]

In a warm house it could evaporate that quickly.
Try and take note of when the smell is about.

Causes of loss of water seal
Syphonage
Evaporation
Leaking trap
Wick effect
Wind
Positive pressure>This is a 'bogey' because it could blow the water seal up into the basin yet fill the room with air from the drains and the water runs back into the trap

Do you think it's maybe the washing machine or dishwasher that is causing the water to be syphoned from the trap??

Thats your number 1 suspect

Is there nothing I can really do then, apart from running the tap every so often to ensure the trap always has water in it??

Anti syphon trap, Hepv0 valve or give the basin its own waste pipe without connecting anything else into it.