At shortly after 9.00am on Friday, the 5th of July 1991, Harry popped back into normal reality in amongst the trees of St James Park near the Park Office, hidden from sight from mundanes. A quick check ensured he was wearing the disguise he and Myrrdin had worked out to get him inside 10 Downing Street; a school uniform with a school satchel holding the necessary documents slung from his left shoulder.
He looked around, oriented himself with the direction he needed to go and set off on foot. He did not see a team of wizards suddenly apparate near to the spot he arrived.
Crossing Horse Guards Road he entered Downing Street from the west end and leisurely walked along the footpath towards the doors of Number 10. His timing needed to be exact for this part.
Just as he was within a hundred yards of the door he spied his ticket into Number 10 just ahead. The class of nine and ten year olds from Years 4 and 5 at St Tristan's Primary School were clustered around an already harassed teacher waiting to start their school tour of the Prime Minister's Office, which would begin momentarily. He simply approached the group from the rear and, blending in, cast a Notice-me-Not Charm on himself using a bit of wandless magic. Anyone looking at him would simply see a normal school student but wouldn't notice that no one knew who he was.
The door opened and a young lady stepped out. She gestured to the teacher to follow her and went back inside.
"Alright, then, children. Here we go," the teacher called out over her charges. "Remember. Best behaviour now. The Prime Minister and his staff are very important people, and you're not to bother them or get in their way." Then she ushered her young charges in through the door.
This was necessary for Harry to get past the Office's main security checkpoint just inside the door as neither Myrrdin nor he, during their research, could discover whether or not mundane security systems could detect someone magically obscured. They didn't want to risk that they could.
Once past security Harry simply dropped back and allowed the group to pull ahead and, with a further piece of wandless magic, altered his Notice-me-Not Charm to a Disillusionment Charm. He also changed his appearance to that of a young man wearing a business suit that looked to be right off a dressmaker's dummy at Saville Row. His school satchel now looked like a business satchel.
He headed directly to the Office of the Prime Minister's Principal Secretary. All he had to do was make sure he didn't bump into, or otherwise come into contact with, anyone along the short walk down a couple of corridors.
Passing a small security desk set up at the bottom end of the last corridor - pausing for a second to make it look like he was flashing the man credentials for the security camera - Harry reached his destination. Thankfully, the door with a small wooden plaque that said 'Lady Muriel Columbus, Principal Private Secretary' was already opened wide and he only had to walk in.
The middle aged lady sitting behind the desk frowning down at papers before her matched the lady he'd seen on Myrrdin's time viewer.
'All good, so far,' he thought.
He glanced out the door to ensure he wasn't seen, dropped the Disillusionment Charm while the lady wasn't looking at him and quietly closed the door.
Turning back around Harry noticed the lady still hadn't paid him any attention, so he quietly said, "Excuse me, Ma'am."
On hearing his voice the lady quickly looked up at him, frowned and said, "What are you doing in here, Sir? You're not supposed to be here."
She started to reach for the telephone sitting on her desk.
Before her hand reached it, and looking her straight in the eye, Harry quietly said, "Merlin Yellow, Ma'am."
She froze.
Lady Muriel Columbus was not a lady easily startled, surprised or shocked. She had served her current Prime Minister since the day he had assumed office less than a year ago. She had also served him for nearly the entire time he served in the Ministry as Foreign Secretary, and then Chancellor of the Exchequer, in the previous Government; and knew the Prime Minister thought of her as a trusted advisor.
As his Principal Private Secretary when her Minister assumed his current office, she was briefed on the magical world. A world she would have flatly refused to believe existed if she wasn't provided proof of its existence in the office through the unadorned door behind her and just off to one side. That incontrovertible proof she still found difficult to accept.
The knowledge of that world wasn't so much Classified as it was so... unbelievable. She knew she'd be removed from her posting if she ever spoke about it. People would think she was 'losing it', as her young grandson would say.
But, as part of that briefing she was told a set of code words that, when someone spoke one of them to her, would mean there was trouble in that other world; and that the person who spoke them would need to see the Prime Minister. 'Merlin Yellow' meant there was an imminent danger in, or originating with, the magical world, and that the person would need to see the Prime Minister immediately.
In shock, Lady Muriel stared at Harry and said, "Ehhhrrr... what?"
That was not something someone with her impeccable manners was normally wont to do, no matter the surprise.