Thank you for your help and replies. 

Unfortunately, despite zooming out, setting the viewport scale and a lot of panning, I couldn't even find a wall as it seem to have enlarged so much. The only way I could see my plan at all was "scale to fit" which put the viewport scale in decimal points.  

It only worked after I changed the page setup settings. For some reason, the scale there was set to something like 1mm = 0.0039 unit.

Once set to 1mm = 1 unit, the viewport scaled properly and my plan is now in 1:25.