Thanks for the info.  Was just looking at the new section as I don't get much of a chance through the week as It's usually getting dark by the time I get home.  Picked up a couple of things today and then noticed the gap between the tiled entrance  and the door and it measures 20mm.   How can this be fixed?

20mm is still ok. Given that it is an external door and if it is a new timber door, then the carpenter might be allowing for some swelling. 

If you don't want anything getting through (dirt/wind), then I would put a rubber strip on the back of the door. It's called a door seal or door sweep. You can get these from the hardware store: http://www.bunnings.com.au/raven-915mm-rp3-door-seal_p4065361

Just concerned of the creepy crawlies coming through the gap.  We're not going to have a security door fitted as the entrance door has leadlight detailing and it's the main entrance feature.  Will look at the door seal/sweep option.  Thank you.

with a 20mm gap I would hope they have left such a big gap to allow for a weather strip because at 20mm the mice will walk straight in without even having to duck under let alone the other creepy crawlies