Can you buy tickets to get into Crufts at the door? Or do I have to pre- order online as we are 1 ticket short.

you can buy them on the door, not sure about the dog doors though, you might have to go to the main entrance.  Probably easier to buy online :)

But on line and get a e ticket so they are sent to your e-mail and you can print them off a day or 2 before hand, thats what we did last year

Some of the dog doors had them but not all from memory as we had to purchase for the kids one year.

If you are entering via the exhibitors' doors and are a pass short, just look out for someone with more dogs than people and ask if you can walk in with them :) I can't see anyone objecting.