Thank you so much for the info 13rebel.
I had always suspected it had a French connection as the grab position indicator is marked with Avant, Milieu and Arriere, and the prize draw has "Tirez" on it.
But I never knew who the original manufacturers were.
I had guessed from the styling and 3 digit Holborn phone number that it was be pre 2nd world war but I could have been wrong.
With the new knowledge I have learned from you, I searched the web and found a couple of sites suggesting manufacture from around 1930 to 1935.
Also makes sense that it would be called a Transporter Bridge as the crane mechanism runs horizontally on an overhead gantry rail system.