Yes under the Public Bodies (Admission to meetings) Act 1960 4(c) it states:
while the meeting is open to the public, the body shall not have power to exclude members of the public from the meeting [and duly accredited representatives of newspapers attending for the purpose of reporting the proceedings for those newspapers shall, so far as practicable, be afforded reasonable facilities for taking their report and, unless the meeting is held in premises not belonging to the body or not on the telephone, for telephoning the report at their own expense ] [;
So yes you should have access at any point of the meeting (unless it is an item to which the public can be excluded). It would be illegal, but not criminal, to prevent you from exercising your rights by locking the door.
I guess they could try and claim that they would have let you in if you knocked, but i doubt any judge would buy that. But unfortunately this is something that would likely never get before a court as there is no one that would be willing to enforce this law.
Would also probably be a breach of the Code of Conduct that requires transparency. Again the MO would be probably be useless in enforcing this though.