Agree with both you and David.
It would be a problem if Sheila escaped.
However there was as much chance of Sheila escaping as June waking Sheila with a scream.
I believe the events on the landing transpired as follows.
1: Nevill calls Jeremy for help from the downstairs kitchen.
2: Sheila enters the main bedroom and begins fireing the five non fatal shots at June Bamber.
3: Nevill abruptly ends the conversation on the phone and runs upstairs.
4: After fireing five shots into June. Sheila realises Nevile is on his way up.
5: Sheila then pivots herself to aim the gun towards the stairs.
6: June then gets out of bed to try and escape via the box room.
7: Sheila then fires the last four shots in the magazine at Nevill.
8: Nevill now injured retreats back to the kitchen and Sheila goes after him.
9: June is now on the other end of the bedroom a few feet from the box room.
10: With Sheila's attention turned to Nevill. June decides to go after her to try and help Nevill.
11: June collapses in shock and dies when she gets to the hallway door.