Tiggerblue don't stress yourself over the Mortgage surveyor -honestly they do very little! He will have a damp meter which he will put against the walls which will give him a reading, look in the loft to check if you have insulation and maybe ask you a few questions about any renovations or additions to the structure you have or have not made. He will 'cast an eye' outside for obvious signs of shifting foundations (very rare) and if yo have a flat roof on any part of the property he may have a closer look at that.
As for the smell under the stairs - I would leave the door open as long as you can and give it a massive spray of neutradol or similar literally as you see him walking to the front door.