Credit checks - generally when are they done?

Hi all
I've just seen this subject mentioned on another thread but am a bit confused (it doesn't take much!) Is there a general rule as to when credit checks are done for mortgage applications? We are still awaiting a DIP (long story, this is the 3rd week). Will we be credit checked before we get this? I thought that you got a DIP and checks were then done after full application.

TIA :)