im not sure on exact details but as far as im aware if you answer the door to a bailiff then they have then got the right to force their way on to your property at any time with or without your presence, but if you dont answer the door to them that first time they cant do anything.
A person working here used to be a bailiff so I've just asked him. shorty87 is right, once you let them in voluntarily they then have the right to force their way in afterwards, whether you are there or not. If you refuse to "invite" them in they cannot enter at any time without you being there.