Well it was defined in 2000 by the Beatson review, but I don't know how much notice a court would take of that.
Doors
6.4 Outer doors: A distinction is made between outer doors and inner doors. The
general rule is that if the occupier has left an outer door so that the premises is
accessible through it to people who have business there, then the distrainor can
open it in the ordinary way.74 It follows that a distrainor cannot generally break an
outer door open.75 He or she can, however, go through it if it is fastened to keep
the door closed (but not to keep people out).76 It is therefore permissible to push the door,77 to lift a latch, to draw back a bolt, or to turn a key to get the door
open.78
6.5 Once lawfully on the premises, a distrainor can also open another outer door to let
in someone who is lawfully assisting with the operation.79 Similarly, a distrainor
who has lawfully entered the premises can break open an outer door so that he or
she can get the goods out.80
6.6 Inside doors: Once inside the building, the distrainor can break open any inner
door to find and remove goods81 or to escape (or rescue someone else) if locked
in.82 He or she does not have to make a demand before breaking open an inner
door.83
6.7 The distinction between inner and outer doors: This can be difficult if the debtor
only occupies some rooms of the premises as in lodgings or a “bedsit”
arrangement, or in an apartment block. The position of a room in a shared house
was considered in Lee v Gansel,84 where the defendant occupied four rooms in a
house, each of which had a door leading on to a common hallway. The Court held
that the outer door was the entrance to the building.85 In the case of a flat in an
apartment block, however, it is arguable that the outer door is the door to the flat.
This professor did quite a good job of defining the law, with a view to proposing changes to it.
As you can see, it's a bit grey - sounds like a question for TT methinks
Chris.
Bailiffs and HMO? 
