Well, I was one of the lucky few that DID manage to go on the tour - and all thanks must go to the Network Rail staff for organising it, and nothing to the OD folk who managed to turn ticketing into a rammy, and even with a confirmed EventBrite booking, this didn't manage to filter through to Station Staff (long story).
The so-called 'lost platforms excited me, until I realised in my youth when plunking school, I went down to the Low Level and recall seeing no tracks, but two island platforms.... and daylight! The tenement at the corner of Hope Street and Argyle Street either fell down or was otherwise removed, leaving a vacant site and a steep gradient from street level down to what would have been basement/foundation level. I believe it allowed up to 40-50 cars to park, BUT you could look across to the two island platforms and the tunnels heading East/West (now the Argyle line).
The renovation of the Low Level only refurbed ONE of the island platforms, the remaining was then hidden behind a wall built to contain just the two tracks. By this time Westergate had replaced the vacant ground but supposedly did not encroach further.
The impression I got from the tour(and I could be mistaken due to information overload - was that there was TWO tunnels adjacent, one the original Central Line (now Argyle) and the 'hidden' one, that headed off in the direction of the 'Royal infirmary'. However. as the LL line was constructed by cut and cover and follows the alignment of Argyle Street, there is not enough room for two running tunnels.
Can anyone shed any light on this?
https://dl.dropboxusercontent.com/u/3205049/DSC00232.JPGThis picture is of the hidden platform at Central Station low level (looking east) the island platform edge can be clearly seen on the left.
As for the platforms supposedly being '20 & 21' - not so. These are numbers counting the iron support pillars - in my picture, which is of the reverse view shows different numbers.