I have recently noticed a cat wee smell in my hall, we don't have a cat and never have had, which comes and goes. We thought it was from a rain water drain just outside the front door but having checked that that isn't blocked and the other drains are running clear, we are at a total loss as to what is causing this smell. It is worse when it rains which directs us back to this drain but aren't sure what to do next. Has anyone any ideas?

It doesn't have to lock in with the rain. For example, Sewer gas ,(and other bad smells will raise with warm weather and decend with rain or colder weather.
It could be a open vent in the attic or a leaky vent in the wall.
In this case your nose is the best detective . Locate the spot where the smell's the strongest that's where to start checking. Good luck, Tom