Mike Hodgson wrote: " I first encountered Platform 9 3/4 as a simple sign on the outside of the suburban station (the platform 9 side) before the rebuilding of Kings Cross.
I have been asked a couple of times by Japanese tourists to take their picture posing by it!
After rebuilding it is now a money making venture called the Harry Potter Shop in the big new concourse (on the other side of what had been the outside wall of the main station). It does have a postcode : London • N1 9AP, so as not to be confused with the railway station, N1 9AL .... "
Apart from Mike's first mentioned location of the "Platform 9 ¾ " sign, and its current one, for quite a while it moved around quite a bit, no doubt to the requirements of the then progressing station rebuilding, and I recall it, and its accompanying brickwork-embedded rear half of a luggage trolley, fixed in at least two other locations (I think it was actually three in total) along the way, one of which was in the walkway between platforms 8 & 9 under the West Side offices, with another being (rather unrealistically) in a wooden hut outside the Euston Road end of the East Side offices, near York Way.
By the time of the London 2012 Olympics it had arrived at its current location but was free to access and have photos taken at, with the oft-necessary queueing being organised, unlike now, by station staff, and having no connection with what is now the adjacent Harry P. shop which then was a small independent bookshop.
Incidentally I believe the sign's current site is on
the outside of what had previously been the exterior wall of the station ; either that or it's in a similar position on the outside of the style-matched new part of the building constructed to finally fill-in the West Side offices' 'bomb gap' of 1941.
And when I worked at KX, I'm sure that the postcode was quoted as N1 9AP : Don't recall N1 9AL at all : Perhaps they have been changed in 28 years !